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+*** START OF THE PROJECT GUTENBERG EBOOK 75519 ***
+
+Transcriber’s notes:
+
+The text of this e-book has been preserved as in the original, except
+for the correction of a single typo (ests → test), the insertion of a
+missing footnote anchor and the shifting of footnotes below the relevant
+paragraph.
+
+Italic text is denoted by _underscores_, and bold text by =equals= symbols
+
+
+
+
+                        THE HUNTERIAN LECTURES
+
+                                  ON
+
+                  COLOUR-VISION AND COLOUR-BLINDNESS
+
+
+
+
+                                  THE
+
+                          HUNTERIAN LECTURES
+
+                                  ON
+
+                           COLOUR-VISION AND
+                           COLOUR-BLINDNESS
+
+
+      _Delivered before the Royal College of Surgeons of England
+                    on February 1st and 3rd, 1911_
+
+
+                                  BY
+
+                     PROFESSOR F. W. EDRIDGE-GREEN
+
+                        M.D.Durh., F.R.C.S.Eng.
+
+                     BEIT MEDICAL RESEARCH FELLOW
+
+
+               KEGAN PAUL, TRENCH, TRÜBNER _&_ Co., Ltd.
+                     43 GERRARD STREET, LONDON, W.
+                                 1911
+
+
+
+
+PREFACE
+
+
+As there are many who are interested in the subject of vision and
+colour-blindness who are not acquainted with the structure of the eye,
+I will give a few details so that these persons may be able to consider
+the problem from the point of view of these lectures.
+
+The eye is very similar to a photographic camera, and an actual image
+is formed on the back of the eye just as it is on the plate of the
+photographic camera or on the view-finder. The eye possesses a lens and
+also an iris which acts as an adjustable stop and regulates the size
+of the pupil. The membrane at the back of the eye upon which the image
+is formed is called the retina. The retina has several layers, but the
+sensitive layer consists of two elements called, from their shape, rods
+and cones. The problem therefore which has to be considered is, how is
+the light which forms the image on the sensitive layer of the retina
+transformed into visual impulses?
+
+Those who are interested in the subject will find further details in my
+book on _Colour-Blindness and Colour-Perception_ in the International
+Scientific Series. In that book there are three plates which show how
+the colour-blind see colours.
+
+I have been annoyed to find that unauthorised persons have made
+lanterns professing to be mine but grossly inaccurate. The sole makers
+are those mentioned on page 53 in this book.
+
+  F. W. Edridge-Green.
+
+  The Institute of Physiology,
+  University College,
+  Gower Street, London.
+
+
+
+
+CONTENTS
+
+
+LECTURE I
+
+THE THEORY AND FACTS OF COLOUR-VISION AND COLOUR-BLINDNESS
+
+                                                                   PAGE
+
+  THE VISUAL PURPLE THE ESSENTIAL FACTOR IN VISION                   11
+
+     1. Anatomical Evidence                                          11
+
+     2. Physiological Analogy with other Body Cells                  12
+
+     3. The Relation between the Foveal and the Extra-Foveal
+          Regions                                                    13
+
+     4. The varying Sensibility of the Fovea                         13
+
+     5. Chemical Analogy                                             14
+
+     6. Disappearance of Lights falling upon Fovea                   15
+
+     7. Illusion of Moving Light                                     16
+
+     8. Purple After-Image                                           17
+
+     9. Currents seen in the Field of Vision not due to the
+          Circulation                                                17
+
+    10. Pressure Figure                                              19
+
+    11. Macular Star                                                 19
+
+    12. Entoptic Appearance of Cone Mosaic                           20
+
+    13. Visual Acuity                                                21
+
+  THE EVOLUTION OF THE COLOUR-SENSE                                  26
+
+  THE FACTS OF COLOUR-BLINDNESS                                      34
+
+     1. Defects of Light-Perception                                  35
+
+     2. Defects of Colour-Perception                                 38
+
+  THE TWO MAIN VARIETIES OF COLOUR-BLINDNESS:
+
+     1. Dichromic Vision                                             40
+
+     2. Trichromic Vision                                            42
+
+
+  LECTURE II
+
+  THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW
+
+     1. Object of a Test for Colour-Blindness                        44
+
+     2. The Requirements of a Test for Colour-Blindness              45
+
+     3. Persons to be Excluded                                       47
+
+     4. The Construction of a Test for Colour-Blindness              48
+
+     5. The Lantern Test                                             53
+
+     6. Other Tests for Colour-Blindness                             66
+
+
+
+
+LECTURE I
+
+_Delivered on February 1st_
+
+
+GENTLEMEN,--Colour-blindness is not a good name for the condition
+to which it is applied, and still worse is the use of the term
+red-blindness or green-blindness. In the majority of cases of
+colour-blindness there is no blindness to colours in the ordinary
+acceptation of the term; a green, red, or yellow light produces a very
+definite sensation of colour. Those who confuse red and green do so,
+not because they see red as green or green as red, but because both
+give rise to a similar sensation of colour. The word light must be
+used in the sense of referring to those waves which excite the organ
+of vision. Because two stimuli excite a sensation of light, it does
+not follow that they are similar. We cannot, for instance, distinguish
+by the eye polarised light from non-polarised light. We have to
+distinguish between the physical stimuli by their physical properties
+apart from their effect on the organ of vision. I propose to divide the
+subject into two parts, and in this lecture to deal with the theory and
+facts of colour-vision and colour-blindness, and in the second lecture
+with the detection of colour-blindness from a practical point of view.
+
+
+
+
+I. THE THEORY AND FACTS OF COLOUR-VISION AND COLOUR-BLINDNESS
+
+
+The following is the theory which I have propounded in order to explain
+vision and colour-vision. A ray of light impinging on the retina
+liberates the visual purple from the rods and a photograph is formed.
+The rods are concerned only with the formation and distribution of
+the visual purple, not with the conveyance of light-impulses to the
+brain. The ends of the cones are stimulated through the photo-chemical
+decomposition of the visual purple by light (very probably through the
+electricity which is produced), and a visual impulse is set up which is
+conveyed through the optic-nerve fibres to the brain. The character of
+the stimulus differs according to the wave-length of the light causing
+it. In the impulse itself we have the physiological basis of the
+sensation of light, and in the quality of the impulse the physiological
+basis of the sensation of colour. The impulse being conveyed along
+the optic nerve to the brain, stimulates the visual centre, causing
+a sensation of light, and then passing on to the colour-perceiving
+centre, causes a sensation of colour. But though the impulses vary
+in character according to the wave-length of the light causing them,
+the retino-cerebral apparatus is not able to discriminate between the
+character of adjacent stimuli, not being sufficiently developed for the
+purpose. At most, seven distinct colours are seen, whilst others see
+in proportion to the development of their colour-perceiving centres,
+only six, five, four, three, or two. This causes colour-blindness, the
+person seeing only two or three colours instead of the normal six,
+putting colours together as alike which are seen by the normal-sighted
+to be different. In the degree of colour-blindness just preceding
+total, only the colours at the extremes of the spectrum are recognised
+as different, the remainder of the spectrum appearing grey. Though my
+own opinion is that the ordinary form of congenital colour-blindness
+is caused by a defective development of the portion of the brain which
+has the function of the perception of colour, we must not exclude any
+portion of the retino-cerebral apparatus, defect of which would have
+exactly the same result. It will be noticed that the theory really
+consists of two parts, one concerned with the retina and the other with
+the whole retino-cerebral apparatus. I shall in these lectures use the
+word cerebral in this sense. I am not aware of a single fact which does
+not support this theory, and I have used it to predict facts which have
+subsequently been rediscovered by others and now form a part of our
+common knowledge.
+
+
+THE VISUAL PURPLE THE ESSENTIAL FACTOR IN VISION
+
+I will now state very briefly the evidence which supports the view that
+the visual purple is the essential factor in the retina which enables
+it to transform light into visual impulses.
+
+1. _Anatomical._--In the fovea of the retina only cones are to be
+found. Immediately external to this each cone is surrounded by a ring
+of rods. The number of rings of rods round each cone increases as the
+periphery is reached. The outer segments of the cones are situated in
+a space which is filled with fluid. The external limiting membrane
+retains this fluid in its place. I find[1] four depressions or canals
+which lead into the larger depression of the external fovea. These
+canals appear to have smaller branches, and serve to conduct the
+visual purple into the part of most acute vision. The cones which are
+present in the fovea have very long outer segments which would present
+a greater surface for photo-chemical stimulation. The visual purple
+is only to be found in the rods and not in the cones. I determined to
+ascertain whether the visual purple could be seen between the cones
+in the fovea. I have examined under the microscope the retinas of two
+monkeys which had been kept previously in a dark room for forty-eight
+hours. The yellow spot was the reddest part of the whole retina, and
+the visual purple was seen to be between and not in the cones.[2]
+
+[1] _Journal of Physiology_, vol. xli, p. 274.
+
+[2] _Transactions of the Ophthalmological Society_, 1902, p. 300.
+
+2. _Physiological analogy with other body cells._--It is far more
+probable that the rods should produce a secretion which would affect
+other cells rather than themselves. The liver cells do not form bile
+in order to stimulate themselves, and the internal secretions are
+produced to affect other parts of the body. I am not aware of a single
+instance in which a cell produces a secretion which has the function
+of stimulating the cell producing it. The visual purple is regenerated
+in the rods by the pigment cells in connection with them.
+
+3. _The relation between the foveal and the extra-foveal regions._--As
+the fovea only contains cones, if any of the older theories of the
+relative functions of the rods and cones were true we should expect
+to find qualitative differences between the foveal and extra-foveal
+regions. This is not the case, but as we should expect if the visual
+purple were the visual substance, all the phenomena which have been
+attributed to the visual purple should be found in the fovea. Von
+Tschermak, Hering, Hess, Garten and others have found the Purkinje
+phenomenon, the variation in optical white equations by a state of
+light and dark adaptation, the colourless interval for spectral lights
+of increasing intensity, and the varying phases of the after-image in
+the fovea only gradually diminished.
+
+4. _The varying sensibility of the fovea._--The fovea is in some
+conditions the most sensitive part of the whole retina, and with other
+conditions the least. Helmholtz has recorded some of these facts
+and regarded them as quite inexplicable. We have, however, an easy
+explanation of the facts on the assumption that when there is visual
+purple in the fovea this is the most sensitive part of the whole
+retina, but when there is none there time must elapse before it can
+diffuse into the spot, and in the meantime it is insensitive to light.
+I have devised several experiments which show the visual purple flowing
+into the foveal region. The following simple experiment shows this very
+well. If on awaking in the morning the eyes be directed to a dull
+white surface, as for instance the ceiling, the region of the yellow
+spot will appear as an irregular black spot, and light will appear to
+invade this spot from without inwards. If the eyes be now closed and
+covered with the hands, purple circles will form round the centre of
+the field of vision and gradually contracting reach the centre. When
+the circle reaches the centre it breaks up into a star-shaped figure
+and becomes much brighter. It then disappears and is followed by
+another contracting circle. Now it will be noticed that if one eye be
+opened when the circle has broken up, a brilliant rose-coloured star
+much brighter than any other part will be seen in the centre of the
+field of vision. This has the exact hue of the visual purple. If we
+wait until the star has disappeared before opening an eye, the macular
+region appears as a black spot as before. This conclusively shows that
+the central portion of the retina is sensitised from the peripheral
+portions.
+
+5. _Chemical analogy._--The visual purple gives a curve which is very
+similar to that of many other photo-chemical substances. We know that
+with photo-chemical substances the chemical effect is not proportional
+to the intensity of the light. That is, a different curve is obtained
+with weak light from that which is formed with light of greater
+intensity. It is reasonable, therefore, to suppose that the visual
+purple which is formed by the pigment cells under the influence of a
+bright light would be somewhat different in character from that which
+is formed in darkness. Again, from the chemical analogy which I have
+just given, even if the visual purple were of the same character, we
+should not expect similar curves with different intensities of light.
+It is probable that both factors are in operation. This deduction
+gives an explanation of the Purkinje phenomenon, or the fact that when
+the eyes are adapted to darkness the point of greatest luminosity is
+shifted more towards the violet end of the spectrum. Some dichromics
+who have shortening of the red end of the spectrum have a luminosity
+curve which is very similar to that of a normal-sighted person with a
+spectrum of lesser intensity. We have only to assume in these cases
+either that the receiving nervous apparatus is less responsive, or
+that the visual purple formed at one intensity of light is similar to
+that formed at a lower intensity by a normal-sighted person. We also
+have an explanation of other conditions, such as erythropsia, or red
+vision, white objects appearing more or less red. If we suppose that
+the eye has remained in a state of light adaptation, the visual purple
+produced being more sensitive to the red rays, objects appear of this
+colour. As we should expect, erythropsia is frequently associated
+with hemeralopia, or difficulty in seeing in the twilight, the eyes
+being adapted to light and not to darkness. In green vision the eyes
+have probably remained in a condition of more or less adaptation to
+darkness, and are therefore more sensitive to the green rays.
+
+6. _Disappearance of lights falling upon fovea._--If the cones are not
+sensitive to light, a ray of light falling upon the fovea alone and not
+upon the adjacent portion of the retina containing rods should produce
+no sensation of light, provided that there is not already any visual
+purple in the fovea. It has been known to astronomers for a long time
+that if a small star in a dark portion of the sky be steadfastly looked
+at, it will disappear from view, whilst other stars seen by indirect
+vision remain conspicuously visible. The following simple experiment
+shows the same thing. If a piece of black velvet about three feet
+square on a door have a pin put in the centre, and the source of light
+be behind the observer, the pin will be brightly illuminated; and on
+looking at it (the observer not being too close) and keeping the eye
+quite still, the pin will disappear, the visual substance diffused
+into the fovea centralis being used up and not renewed. When viewed by
+indirect vision it is impossible to make it disappear in this way. When
+I have taken great care to have very dark surroundings and have used
+only one eye, I have made moderately bright lights disappear in this
+manner. These facts have been attributed to a defective sensibility
+of the fovea for feeble light. The important point, however, that the
+light is at first most clearly seen by the fovea and only subsequently
+fades, has been overlooked. If these facts were due to a defective
+sensibility of the fovea, the star or light would not be visible at
+first.
+
+7. _Illusion of moving light._--If a small light be looked at fixedly
+in a dark room, it will appear to move until it comes apparently so
+close that it could be grasped. The reason of this is that the eye
+moves so that the light falls upon a more peripheral part of the
+retina. I find that the movement takes place as if some photo-chemical
+substance acted under the influence of gravity. For instance, when
+standing the light appears to travel upwards; resting the head on one
+side, it appears to travel in the opposite direction. The light appears
+as if we were looking straight at it, and the eye, which is covered up,
+remains directed straight at the object. When the second eye is opened
+two images of the light are seen, and the image which is seen with the
+periphery of the first eye rapidly coalesces with that seen directly by
+the second eye.
+
+8. _Purple after-image._--A positive after-image of a purple (rose)
+colour can be obtained after white light or any spectral colour. It
+will be noticed that when there is little light during the subsequent
+observation the colour of an after-image inclines to blue or green,
+when there is more light towards purple or red.
+
+[Illustration: Fig. 1.]
+
+9. _Currents seen in the field of vision not due to the
+circulation._--It occurred to me that if there were canals in the
+retina which promoted the easy flow of the visual purple into the
+fovea, we ought to obtain evidence of the currents flowing along these
+channels entoptically. I found that this was the case, and that the
+currents could be seen in numerous ways.[3] If one eye be partially
+covered with an opaque disc whilst both eyes are directed forwards in
+a not too brightly illuminated room, and special attention be paid to
+the covered eye, an appearance of whirling currents will be seen with
+this eye (see Fig. 1). These currents appear to be directed towards the
+centre, and have a very similar appearance to a whirlpool which is fed
+by four main branches. These again are fed by smaller branches which
+continually change their paths. On closing both eyes all the portion
+in which the whirling currents are seen appears as dull purple. These
+currents cannot be due to vessels, because we know that the centre
+of the retina corresponding to the point where the greatest movement
+is seen is free from vessels. The appearance is also very different
+from that of the movement of blood in vessels. The currents can also
+be seen in the light, in the dark, through yellow-green glass, and
+with intermittent light. The main branches form a star-shaped figure
+with four rays. The currents carry the visual quality, colour, and
+brightness from whence they come into an after-image. They also tend
+to move an after-image towards the centre. The currents behave as if
+they ran in definite channels, but could also overrun, on any further
+stimulus, the banks of the channels. For instance, a thin, bright line
+with a little more light appears as a broad band, and the central star
+figure will enlarge into a rhomboid, oval or disc. Movements of the
+eyes affect the broad currents in the outer part of the field of vision.
+
+[3] _Journal of Physiology_, vol. xli, p. 269.
+
+10. _Pressure figure._--Pressure on the front of the eye causes the
+star-shaped figure to be seen, and this changes into a rhomboid with a
+little more pressure.
+
+11. _Macular star._--It occurred to me that we ought to obtain evidence
+of the canals in the retina in cases where the outflow from the retina
+is obstructed, as by tumour. I find this is the case; the star-shaped
+figure given by Sir Victor Horsley in his paper on tumour of the
+frontal lobe[4] is almost exactly the same as that seen subjectively.
+
+[4] _British Medical Journal_, 1910, p. 556.
+
+[Illustration: Fig. 2.]
+
+12. _Entoptic appearance of cone mosaic._--Appearances corresponding
+to the cone mosaic of the retina may be seen in several ways[5] (see
+Fig. 2). The appearance seen corresponds to the cone distribution of
+the retina as viewed from its outer side, the portions occupied by rods
+appearing as dark spaces.
+
+[5] _Journal of Physiology_, vol. xli, p. 226.
+
+13. _Visual acuity._--Visual acuity is most acute with the fovea, and
+diminishes from within outwards. It corresponds very fairly with the
+cone distribution of the retina. On the other hand, there is not one
+single fact which points to the rods as being light-sentient organs.
+This is well recognised by those best qualified to judge.[6] I could
+give many more facts in support of the view that the visual purple is
+the visual substance, and I have not yet had brought to my notice any
+fact which is not readily explicable on that hypothesis. There may be
+other photo-chemical substances in the retina, but there is not the
+slightest evidence that such is the case. I regard the visual purple as
+the sole visual substance. We could, of course, split the visual purple
+into innumerable simpler photo-chemical substances, each of which has
+its own absorption curve, having its maximum in some particular part of
+the spectrum. It is difficult to say at present exactly how the visual
+purple acts as a stimulus transformer, but this is because so many
+plausible hypotheses immediately occur to us. It is very probable that
+light acting upon the visual purple is, according to its wave-length,
+absorbed by particular atoms or molecules, the amplitude of their
+vibrations being increased. These vibrations may cause corresponding
+vibrations in certain discs of the outer segments of the cones, which
+seem especially constructed to take up vibrations. We know that when
+light falls on the retina it causes an electric current. We know how
+the telephone is able through electricity to convey waves of sound,
+and something similar may be present in the eye, the apparatus being
+especially constructed for vibrations of small wave-length. The current
+of electricity set up by light may cause the sensation of light, and
+the vibrations of the atoms or molecules the sensation of colour.
+
+[6] _Nagel. Physiol. des Menschen_, vol. iii, p. 107.
+
+In all vital processes there is a condition of katabolism or chemical
+change in the protoplasm, and an anabolic or building-up process, in
+which the protoplasm is restored to its normal state. We have therefore
+to consider two definite processes in the visual purple--namely, a
+breaking down of the visual purple photo-chemically by light and its
+restoration by the pigment cells and rods. Under ordinary conditions
+of light, and during the whole of the daytime, the visual purple is
+continually being bleached and reformed. It is obvious, therefore,
+that when the eye has been kept in the dark and is then exposed to
+light, an observation taken immediately will not be comparable with
+one taken a few seconds afterwards, because in the first observation
+we have only to consider the katabolic change; whilst in the second
+observation the anabolic change has to be considered as well, as the
+visual purple has to be reformed for subsequent seeing. There appears
+to be very little evidence in ordinary circumstances of this anabolic
+process; for instance, if we fatigue the eye with sodium light in a
+dark room, and then immediately examine a spectrum, we find that
+though all the yellow has disappeared there is no increase in the blue;
+in fact, the blue seems rather diminished than otherwise. Again, there
+is not the slightest diminution in either the red or green, showing
+conclusively that yellow cannot be a compound sensation made up by a
+combination of red and green. We must therefore explain in another way
+the apparent trichromatism of normal colour-vision, which is so well
+known to every photographer, especially those who are concerned with
+colour photography. If my theory of the evolution of the colour-sense
+be the correct one, and we have cases of colour-blindness corresponding
+to every degree of the evolutionary process, we have an explanation
+of the facts. In past ages all saw the rainbow made up of only three
+colours--red, green, and violet. When a new colour (yellow) appeared
+between the red and green, it is obvious that a mixture of red and
+green would give rise, not to red-green, but to the colour which had
+replaced it--namely, yellow. The retina, therefore, corresponds to a
+layer of photo-chemical liquid in which there are innumerable wires
+each connected with a galvanometer. When light falls upon a portion of
+this fluid the needle of the galvanometer corresponding to the nearest
+wire is deflected. The wires correspond to the separate fibres of the
+optic nerve, and the galvanometers to the visual centres of the brain.
+
+Cases of colour-blindness may be divided into two classes, which are
+quite separate and distinct from each other, though both may be
+present in the same person. In the first class there is light as well
+as colour loss. In the second class the perception of light is the
+same as the normal-sighted, but there is a defect in the perception
+of colour. In the first class certain rays are either not perceived
+at all or very imperfectly. Both these classes are represented by
+analogous conditions in the perception of sounds. The first class
+of the colour-blind is represented by those who are unable to hear
+very high or very low notes. The second class of the colour-blind is
+represented by those who possess what is commonly called a defective
+musical ear. Colour-blind individuals belonging to this class can be
+arranged in a series. At one end of this series are the normal-sighted,
+and at the other end the totally colour-blind. The colours appear
+at the points of greatest difference, and I have classified the
+colour-blind in accordance with the number of colours which they see in
+the spectrum. The normal-sighted may be designated hexachromic; those
+who see five colours, pentachromic; those who see four, tetrachromic;
+those who see three, trichromic; those who see two, dichromic; and
+those who see none, totally colour-blind. There are many degrees
+included in the dichromic class. There may or may not be a neutral
+band, and this is widest in those cases approaching most nearly to
+total colour-blindness. I have recorded a case of a patient who was
+colour-blind with one eye.[7] It is an interesting fact that for
+form vision the colour-blind eye was much the better of the two, and
+he could recognise fine lines in the spectrum with this eye which
+were not visible to the other. He saw the two ends of the spectrum
+tinged with colour and the remainder grey. It will be noticed that
+his colour sensations were limited to the extreme red and the extreme
+violet--namely, those colours which present the greatest physical
+contrast to each other. Neither the red nor the violet appeared of the
+nature of a primary colour, but gave the impression that they were
+largely diluted with grey. A theory of colour-vision must account for
+a case of this kind, and also for the other varieties and degrees of
+colour-blindness. The trichromic are a very important class, and any
+theory must account for the fact that they see yellow as red-green, and
+blue as violet-green. As we should theoretically expect, when there is
+shortening of the spectrum the centres of the colours are moved towards
+the unshortened side.
+
+[7] “Colour Blindness and Colour Perception,” _International Scientific
+Series_, p. 196.
+
+I will now show on the screen some representations of pictures painted
+by colour-blind persons. The upper picture is the copy, and the one
+below is the one which has been painted by the colour-blind artist.
+He has been given a selection of colours on plates, and from them
+has selected the one which he thought appropriate in each case. It
+will be noticed that the mistakes made are characteristic of the
+colour-perception of the person painting them. Whenever I show these
+pictures, I am asked why it is that these characteristic mistakes
+should be made, and that the true colour of the object is not used
+instead? This undoubtedly would be the case if the artist were allowed
+to match the colours by directly comparing them. But he is not able to
+do this; he looks at the copy and decides upon the colour of an object,
+and then looks for the colour with which to paint it.
+
+A man rarely uses a hue which he does not see as a definite colour, and
+therefore it has been quite possible for me to pick out those who are
+more or less colour-blind in the exhibitors of the picture gallery. For
+instance, if a trichromic have to paint a yellow object he will decide,
+after looking at it, whether it be a red or green in his estimation,
+and represent it accordingly. He will be greatly influenced by the
+nature of colours in its immediate proximity, because simultaneous
+contrast is increased in the colour-blind. Thus he will certainly
+represent a yellow which is adjacent to a red as green, and a yellow
+which is adjacent to a green as red.
+
+
+THE EVOLUTION OF THE COLOUR-SENSE
+
+There can be no doubt that an evolution of the colour-sense has taken
+place: the only point is how and when did this occur. It is obvious
+that in those low forms of animal life in which the most rudimentary
+sense of sight exists there can be no sense of colour. The animal which
+can only perceive light and shade can only discriminate in a rough way
+between varying intensities of the stimulus. It is obvious, therefore,
+that the sense of light must have been developed first and then the
+sense of colour. The sense of sight must have been first developed
+for those waves which produce their maximum effect upon the sensitive
+protoplasm. The next process of development would be for the protoplasm
+to become sensitive to the waves above and below those which produced
+the primary stimulus. In the physical stimulus which produces the
+sensation of light there are two factors to be considered, the length
+of the wave and its amplitude: the greater the amplitude within certain
+limits the greater the intensity of the sensation. The wave-length of
+the physical stimulus is the physical basis of the sensation of colour.
+How did the sensations of colour first arise? Let us suppose that the
+physiological effect of the physical stimulus differed according to the
+wave-length of the physical stimulus.
+
+Let us consider that the eye has reached a stage in which it has
+become sensitive to a fair range of the spectral rays; that is to
+say, evolution has proceeded to the extent of making the protoplasm
+sensitive to rays of light considerably above and below those which
+first caused a sensation of light. We now have an eye which is
+sensitive to the greater part of the rays which form the visible
+spectrum. It is, however, an eye which is devoid of the sense of
+colour; no matter from what part of the spectrum the rays be taken the
+only difference which will be appreciated will be one of intensity.
+I however mentioned that in the physical stimulus there were two
+variables, wave-length and amplitude of the wave. Let us now suppose
+that a fresh power of discrimination was added to the eye and that it
+became able to discriminate between different wave-lengths of light.
+What would be the most probable commencement of development of the
+sense of colour? Undoubtedly to my mind the differentiation of physical
+stimuli which were physically most different. That is to say, the eye
+would first discriminate between the rays which are physically most
+different in the visible spectrum, the red and the violet, that is
+presuming the eye had become sensitive to this range. It is probable
+that it had not, and there has been a steady evolution as to the extent
+of the spectrum perceived as well as to colour. We have examples of
+this in those cases of defective light-perception in which there is
+shortening of the red or violet end of the spectrum.
+
+Let us now work out the evolution of the colour-sense on the assumption
+that the rays which are physically most different, namely, red and
+violet, were those which were first differentiated. We know that the
+various rays differ in their effects on various substances; the red
+rays are more powerful in their heating effects, whilst the violet
+rays are more active actinally, as is well known by the readiness with
+which they act upon a photographic plate, which is scarcely affected by
+red light. We should now have an individual who would see the spectrum
+nearly all a uniform grey of different degrees of luminosity, but with
+a tinge of red at one end and a tinge of violet at the other. There is
+a great deal of evidence to show that this is how the colour-sense was
+first developed. For instance, in the degree of colour-blindness just
+preceding total the spectrum is seen in this way. I have also examined
+a woman who became totally colour-blind, apparently through disease
+of the ear. I examined her when she had recovered a certain amount of
+colour sensation; her sensations were confined to the extreme red and
+violet. As the colour sense developed it was not necessary that the
+rays should differ so much in refrangibility before a difference was
+seen, and so the red and violet gradually invaded the grey or neutral
+band, until at a certain point they met in the centre of the spectrum.
+Such cases are called dichromics.
+
+The next stage of evolution of the colour-sense is when the
+colour-perceiving centre is sufficiently developed to distinguish three
+main colours in the spectrum. The third colour, green, appears in the
+centre of the spectrum, that is, at the third point of the greatest
+physiological difference. In accordance with the prediction of the
+theory, I found a considerable number of persons who saw the spectrum
+in this way, about 1·5 per cent of men. The trichromic see three main
+colours in the spectrum--red, green, and violet. They usually describe
+the spectrum as consisting of red, red-green, green, green-violet, and
+violet. They do not see yellow and blue as distinct colours, and are
+therefore in continual difficulty over them. There are very few of the
+tests in general use which can detect them, especially if names be not
+used. They will usually pass a matching test with ease. An examination
+with the spectrum shows that their colour-perception is less than the
+normal in every part, though the curve has the same general shape.
+The three trichromics described in my recent paper[8] on “The Relation
+of Light-Perception to Colour-Perception” each saw ten consecutive
+monochromatic patches in the spectrum instead of the eighteen or
+nineteen seen by those who see six colours in the spectrum. It is easy
+to show that the trichromic are dangerously colour-blind. They will
+mark out with my colour-perception spectrometer a patch containing
+greenish yellow, yellow, and orange-yellow, and declare that it is
+absolutely monochromatic. When tested with coloured lights they find
+great difficulty with yellow and blue. Yellow is continually called red
+or green.
+
+[8] _Proceedings of the Royal Society_, vol. B 82, 1910, p. 458.
+
+There are several other degrees of colour perception, and it may
+be well to say a word or two about them, though I class all above
+the trichromic with the normal-sighted for practical purposes, as
+they are not dangerously colour-blind, and can always, in ordinary
+circumstances, distinguish signal lights correctly. In the next
+stage of evolution four colours are seen in the spectrum, and the
+fourth colour appears at the fourth point of greatest physiological
+difference, namely, at the orange-yellow of the hexachromic or
+six-colour people. These persons I have designated “tetrachromic,”
+because they see four distinct colours in the spectrum, that is, red,
+yellow, green, and violet. They do not see blue as a definite colour,
+and are continually classing blues with greens; they usually prefer
+to call blue, purplish green. In the next stage of evolution there
+appeared those who see five colours in the spectrum--red, yellow,
+green, blue, and violet, blue being now recognised as a definite
+colour. These are the pentachromic group. These people pass all the
+tests in general use with ease. They, however, have a definitely
+diminished colour-perception compared with the normal, or those who see
+six colours in the spectrum. They mark out in the spectrum only fifteen
+monochromatic patches instead of eighteen. They cannot see orange as a
+definite colour; for instance, they can never tell whether a strontium
+light, which is red, or a calcium light, which is orange, is being
+shown them.
+
+In the next stage of evolution orange is recognised as a definite
+colour, and thus we get the hexachromic or normal group, and, as
+we should theoretically expect, the yellow of the pentachromic is
+now split up into two colours--orange and yellow. The last stage of
+evolution which we appear to have reached are those who see seven
+colours in the spectrum, and the additional one is called indigo. These
+constitute the heptachromic group, and this seventh colour appears at
+the exact point at which it should appear, according to my theory,
+namely, between the blue and violet. Persons belonging to this class
+have a marvellous colour-perception and memory for colours. They will
+indicate a certain shade of colour in the spectrum, and then next day
+will be able to put the pointer at precisely the same point, a feat
+which is quite impossible to the ordinary normal-sighted person. They
+see a greater number of monochromatic patches in the spectrum than the
+hexachromic, but the curve has the same form. The marking out of the
+heptachromic does not appear correct to those who see six colours; for
+instance, the blue appears to invade the green, and the indigo does
+not appear a definite colour at all. If, however, we bisect the blue
+of the seven-colour man, and then bisect his indigo, on joining the
+centres we get the blue of the six-colour man, showing most definitely
+that the blue has been split up into two fresh colours. It will be
+noticed that there is room for much further evolution, and we could go
+on splitting up the spectrum indefinitely if only we had the power to
+distinguish these finer differences, but as a matter of fact I have
+never met with a normal-eyed man who could see more than twenty-nine
+monochromatic patches in the spectrum, and there are really millions,
+though by monochromatic patches I do not mean twenty-nine separate
+colours. Not only are all the details of the process of the evolution
+of the colour-sense supported by all the facts that we can obtain
+from literature and museums, but the theory accounts for facts which
+were previously inexplicable. The distinction between light-sensation
+and colour-sensation is explained, and all facts of colour-mixing,
+complementary colours, and simultaneous contrast. We can understand
+how, as in many cases which have been recorded, a man may lose his
+colour-perception and still have an unaltered sense of luminosity and
+visual acuity.
+
+The explanation of complementary colours is a fundamental part of the
+theory. It is obvious that the two colours of the dichromic are only
+recognised as different because they are seen in contrast to each
+other, and that when they are mixed they neutralise each other. It is
+the same with the other colour-sensations, when they are developed they
+replace the colours occupying their positions. Therefore green which
+replaces the grey of the dichromic should be, and is, complementary to
+the other two colour-sensations, red and violet combined. In the same
+way when the yellow sensation replaces the red-green of the trichromic
+it should be possible to compound it of both. Also, when the green
+sensation is in a feeble state of development it will not have the
+value that it has at a subsequent stage, and, therefore, yellow will be
+a much redder colour to those persons than the normal, and in a colour
+match of red and green forming yellow, more green will be required.
+
+Simultaneous contrast is also explained. When two colours are
+contrasted each appears to be a colour higher or lower, as the case
+may be, in the spectrum scale; that is to say, the close comparison
+exaggerates the difference. As the colour-blind have fewer colours,
+simultaneous contrast should be greater with them, and this I have
+found to be the case.
+
+There may be some relation between the monochromatic patches and the
+discs in the outer segments of the cones. These are about sixteen in
+number in the guinea-pig. As in photography, the intensity of the
+light is a very important factor in vision. With colours of moderate
+intensity, the periphery of the retina is found to be colour-blind,
+but this apparent colour-blindness disappears when more intense lights
+are used. A person may have shortening of the spectrum with light
+of moderate intensity, but when the light is increased be able to
+recognise the spectrum to its normal limit. The change in steepness of
+gradation, according to the intensity of the light, is well known to
+photographers. The Purkinje effect, a change in maximum sensitiveness
+of the eye according to the intensity of the light, is, in my opinion,
+a photo-chemical effect. I find that the Purkinje effect is found
+for small portions of the retina if a black object has been situated
+in the corresponding part of the field of vision. The yellow pigment
+which is found in the yellow spot probably acts like the yellow screen
+in photography, which, by absorbing the blue and violet rays of the
+atmosphere, renders visible that which would otherwise be invisible.
+This is further borne out by the fact that hunters in India are able to
+hunt later in the day than usual by using spectacles glazed with golden
+yellow glass.
+
+
+THE FACTS OF COLOUR-BLINDNESS
+
+When we consider the path along which a visual impulse has to pass,
+and that each cell has probably some special function in connection
+with that impulse, it is not surprising that we meet with a large
+number of different defects of colour-perception and light-perception.
+Defects of light-perception are quite distinct from defects of
+colour-perception.
+
+1. _Defects of light-perception._--The person having the defect is
+placed in a similar position to a normal-sighted person with those
+particular rays removed or reduced to the same intensity. Defects of
+light-perception may be caused by absorption or by some defect in
+the visual purple or cerebro-retinal apparatus. The chief defect of
+light-perception which is found is shortening of the red or the violet
+end of the spectrum. Let us consider the influence of a shortened
+spectrum upon colour-vision. The first evident fact is that bodies
+reflecting only light, the rays of which occupy the missing portion of
+the spectrum, appear black.
+
+Nearly all colours are compound; that is to say, the coloured body
+reflects other rays than those of the colour seen. Thus a blue-green
+glass may transmit the green, blue, and violet rays of the spectrum.
+Let us suppose that we have a substance reflecting the green, blue,
+and three-quarters of the violet, the colour of the body to a normal
+person being green. Then if we had another substance which reflected
+the whole of the violet, it would appear blue. But with a person who
+could not perceive the terminal fourth of the violet the colour would
+look exactly the same as the green one, and as he could not distinguish
+between the two he would be in continual difficulty with blues and
+greens. All coloured objects reflecting rays occupying the missing
+portion appear darker than they do to the normal-sighted, and are
+always matched with darker colours belonging to a point more internal.
+Thus a dichromic with a shortened red end of the spectrum matches a red
+with a darker green.
+
+It will be noticed that a shortened spectrum, especially if one
+end only be affected, may interfere very little with the general
+appreciation of shade. If, for instance, we take a case in which the
+red end of the spectrum is shortened, so that only three-quarters
+of the red of the normal-sighted is seen, then all bodies which
+equally reflect or transmit these rays can be correctly compared,
+because a similar portion of light has been removed from each. It is
+only when one colour reflects or transmits the rays occupying the
+shortened portion, and the other does not, that there is any definite
+interference with the appreciation of shade. Again, if neither colour
+reflects or transmits rays occupying the shortened portion of the
+spectrum, there will obviously be no interference with the appreciation
+of shade.
+
+A very common mistake due to shortening of the red end of the spectrum
+is the confusion of pink and blue. If a person with considerable
+shortening of the red end of the spectrum is shown a pink which is made
+up of a mixture of red and violet, the red consisting of rays occupying
+the missing portion of the spectrum, only the violet is visible to him,
+and so the pink appears a violet without a trace of red. This pink is
+therefore matched with a violet or blue very much darker than itself.
+
+Mistakes which are due to shortening of the spectrum may be remedied
+if we subtract the rays occupying the missing portion from the colour
+of confusion. For instance, if we take a blue and a pink which have
+been put together as identical by a person with a shortened red end of
+the spectrum, and look at them through a glass which is opaque to the
+red, but transparent to the remaining rays of the spectrum, both will
+appear alike in hue and shade. A person with considerable shortening
+of the red end of the spectrum will look at a red light (which is so
+dazzlingly bright to a normal-sighted person as to make his eyes ache
+after looking at it closely for a few seconds), at a distance of a few
+inches, and remark that there is nothing visible, and that the whole
+is absolutely black. It is obvious that the light must consist only of
+rays occupying the missing portion of the spectrum. The same remarks
+which I have made for a shortened spectrum apply to cases in which
+there is defect of light-perception through absorption or any other
+cause. The person having the defect is placed in a similar position to
+a normal-sighted person with those particular rays removed or reduced
+to the same intensity.
+
+Another effect of shortening of the spectrum when it is sufficient to
+interfere with the difference-perception which appears to be inherent
+in the central nervous system, is that the colours appear to be moved
+in the direction of the unshortened portion. For instance, we find the
+neutral point of the dichromic, with shortening of the red end of the
+spectrum, in most cases further towards the violet end of the spectrum
+in comparison with a case in which the spectrum is of normal length. In
+the same way a trichromic with a shortened red end of the spectrum has
+the junction of the red and green nearer the violet end than in a case
+where there is no shortening.
+
+The point that I specially wish to emphasise is that, though every
+case in which there is defective light-perception can be explained
+by a defective sensibility to light of certain wave-length, not a
+single case of the very large number of persons that I have examined
+can be explained on the older theories; that is, the defect of
+light-perception cannot be explained on the assumption that there is
+a defect in a light-perceiving substance which is sensitive to rays
+of light from a considerable range of the spectrum. A large number
+of cases in which there is shortening of the red end of the spectrum
+escape detection when only the green test is used, as is usual
+according to Holmgren’s instructions.
+
+2. _Defects of colour-perception._--The colour-blind have a diminished
+hue-perception and see a less number of colours than the normal. All
+the symptoms of colour-blindness are such as we should expect from want
+of development of the retino-cerebral apparatus for the perception
+for colour. This is evident even in the slighter cases which show a
+diminished colour-perception compared with the normal. We find that
+the colour-blind are much more dependent on the luminosity of the
+colour than the normal-sighted; they require a stronger stimulus; they
+fatigue more easily with colours than the normal-sighted; they have a
+more marked simultaneous contrast; the visual angle subtended by the
+coloured object requires to be larger, and they have a very bad memory
+for colours. The diminution of colour-perception with a diminished
+visual angle evidently depends upon several causes. It is very marked
+when there is diminished light-perception for those rays which are
+imperfectly seen. It is also dependent upon certain retinal conditions,
+as in scotoma and allied conditions. There are colour-blind persons,
+however, who are able to recognise colours under as small a visual
+angle as the normal-sighted, and I have examined one dichromic (said
+to be red-blind by a physicist) who recognised red easily through the
+thickest neutral glass of my lantern, and who had no difficulty with
+this colour at a distance.
+
+Apart from any other defect of light or colour-perception, every case
+with which I have met has fallen naturally into one of the classes
+I have given; that is to say, every person is either heptachromic,
+hexachromic, pentachromic, tetrachromic, trichromic, dichromic,
+or totally colour-blind. When I first gave this classification of
+colour-blindness, the facts that I discovered were so at variance with
+those generally stated that it was very difficult for those who were
+not well acquainted with the subject to compare the two sets. The
+general knowledge of the subject has, however, steadily increased, and
+the facts which I had so great a difficulty in getting recognised now
+form part of our common knowledge. It would be well, therefore, to
+describe the two main varieties of colour-blindness which are of chief
+practical importance, and to show the relation which they bear to the
+writings of other persons. These two main varieties have dichromic and
+trichromic vision.
+
+1. _Dichromic vision._--The cases which come under this head form
+the class of the ordinary red-green blind. It is under this head
+that nearly every one of the recorded cases may be classed. Vision,
+as far as colour is concerned, is dichromic, the neutral point being
+situated in the green of the normal-sighted at about λ 500. All the
+colours on the red side tend to be confused with each other; therefore
+red, orange, yellow and half of the green are seen as one colour, the
+remainder of the green, blue and violet as the other. The luminosity
+curve in uncomplicated cases is similar to the normal. There may be
+shortening of the spectrum at either the red or the violet end of
+a varying degree. All degrees of shortening of the red end of the
+spectrum may be found. Dichromics with normal luminosity curves are
+those which were formerly designated green-blind; but this designation
+is not in accordance with the facts, because there is no defect of
+light-perception in the green, and the so-called diagnostic mistakes,
+as, for instance, putting a bright green with a dark red, are not made.
+Cases of so-called red-blind are dichromics with shortening of the red
+end of the spectrum. I have shown that the defective perception of
+the red end of the spectrum will not account for the dichromic vision
+which is found in these cases. We may also meet with shortening of the
+spectrum with otherwise normal colour-perception. We also meet with
+dichromic cases forming a series from almost total colour-blindness
+to those bordering on the trichromic. Any theory must account for the
+fact that there are varying degrees of colour-blindness in dichromic
+vision, and why there is a large neutral band corresponding to the
+colours of the centre of the spectrum in some cases, and in others the
+neutral band is so small that the dichromic cannot mark it out. The
+two colours seen by the dichromic are red and violet, though where no
+distinction is seen between yellow and red, and blue and violet, the
+brighter colour will often be selected; that is why so many dichromics
+say that they see yellow and blue in the spectrum. Those who have had
+practical experience of colour-blindness will know, however, that many
+dichromics make no mistakes with the red test. The following will
+give the normal-sighted the best idea of colour-blindness, and it is
+how the dichromic see the spectrum, and explains why they are able to
+distinguish between colours. Let him regard the dichromic as a man who
+has two colours--red and violet and white. The purest red is at the
+red end of the spectrum; this becomes less and less saturated as the
+violet is approached until the neutral or white point is reached; then
+violet comes into the white, and this increases in saturation to the
+termination of the violet. The ordinary dichromic therefore sees green
+as a much whiter and less saturated colour than red.
+
+2. _Trichromic vision._--These persons see three distinct colours
+in the spectrum--red, green and violet. They describe the region
+intermediate between red and green; that is to say, the orange and
+the yellow as red-green, and blue as violet-green. It will be seen,
+therefore, that their chief difficulty is distinguishing yellows and
+blues. A yellow, for instance, which is situated next to a green will
+be called red, and the same yellow when adjacent to a red will be
+called green. There are various degrees of trichromic vision, varying
+from those who are little better than dichromic to those who are
+tetrachromic. The trichromic rarely find any difficulty with their
+three main colours--red, green and violet.
+
+These cases have been described under the name of anomalous
+trichromatics. This name is one which has been given to those persons
+who in making a match between a yellow corresponding to the sodium
+flame and a mixture of thallium-green and lithium-red make a mixture
+which is different to that of the normal.[9] A man who puts too much
+green in the mixture is called a green anomaly; whilst a man who puts
+too much red in the mixture is called a red anomaly. The red anomaly
+is only a trichromic with shortening of the red end of the spectrum,
+and this may be as extensive as in any case of dichromic vision. I
+have, however, described trichromic cases which had all the symptoms
+attributed to the anomalous trichromatics, but they were not anomalous
+trichromatics, as they made an absolutely normal match.[10]
+
+[9] _Proceedings of the Royal Society_, vol. B 76, 1905.
+
+[10] _Transactions of the Ophthalmological Society_, 1907, p. 255.
+_Proceedings of the Royal Society_, vol. B 82, 1910.
+
+
+
+
+LECTURE II
+
+_Delivered on February 3rd_
+
+THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW
+
+
+I. _Object of a test for colour-blindness._--Tests for colour-blindness
+are of two kinds; namely, those which are used for the purpose of
+ascertaining the special phenomena of colour-blindness, and those which
+are employed when the inquiry is made for some practical purpose. As
+with visual acuity, it is necessary to fix an arbitrary standard. As we
+do not wish to exclude a greater number than is absolutely necessary,
+the object of the test should be to exclude dangerous persons and
+dangerous persons only. These persons may have other duties to perform
+which do not require them to possess a perfect colour-sense. I should,
+however, like to see those persons who are specially qualified for
+a certain position, occupy it, for instance, men who have to keep a
+look-out on our most important ships being selected because of their
+accurate colour-vision and visual acuity. I do not mean that I would
+select only those men and reject the others, but that I should like
+to see a second object of a test, namely, to select those who are
+specially efficient so that the Captain might know on whom to rely in
+conditions of exceptional difficulty.
+
+
+II. _The requirements of a test for colour-blindness._--A test for
+colour-blindness, when it is to be employed for some definite and
+specific purpose, as, for instance, excluding dangerous persons from
+certain callings, should be such as to show definitely that the
+persons rejected are dangerous. It is very useful to demonstrate to
+the men and their fellows that a rejected candidate is dangerous. The
+colleagues of a rejected candidate would refuse to risk their lives
+with a man who before their eyes called a red light, green. I was
+expressing these views when a superintendent of a railway company,
+who is using my lamp, told me that he had adopted this method with
+great satisfaction to himself and to the men. A man, for instance, who
+has been working twenty years on the railway has been rejected for
+colour-blindness. He has complained bitterly to the superintendent,
+at the same time declaring emphatically that he is normal-sighted.
+The superintendent has replied, “You know red?”--“Yes.” “You know
+green?”--“Yes.” “You will therefore agree that if you call green,
+red; or red, green, you ought to be rejected. Bring two or three of
+the other men in with you and I will test you.” The man has readily
+agreed to this. The superintendent has then tested him by asking him to
+name various coloured objects in the room, and knowing by experience
+exactly the coloured objects which are miscalled by the colour-blind
+readily exposes his defect. It is noteworthy that on some occasions a
+colour-blind man has been tested by another person in the same room
+without making any of the mistakes which he subsequently made, because
+none but coloured objects which he could readily recognise were shown
+to him. This is an example of the necessity of a practical knowledge
+of colour-blindness in an examiner. On account of the arrangement of
+signals by sea and land, it is necessary that persons employed in the
+marine and railway services should be able to recognise and distinguish
+between the standard red, green, and white lights in all conditions
+in which they are likely to be placed. An engine-driver or sailor has
+to name a coloured light when he sees it, not to match it. He has to
+say to himself, “This is a red light, therefore there is danger”; and
+this is practically the same as if he made the observation out loud.
+Therefore, from the very commencement we have colour-names introduced,
+and it is impossible to exclude them. The engine-driver is told that
+red is a “danger” signal, green a “caution” signal, and white an “all
+right” signal. Therefore, it is absolutely necessary that he should
+know the meaning of these colour-names. A test should be such as to
+make it impossible for the examinee to be coached through it. This is
+one of the most important requirements of a test for colour-blindness
+and one that is rarely fulfilled. Nearly every one of the tests in
+general use fail on this account.
+
+A test should be one which can be carried out as rapidly as is possible
+with absolute efficiency; of two equally efficient tests the one which
+takes the least time must be selected. A test, therefore, should have
+no unnecessary details which though of theoretical interest are not
+concerned with the object in hand. The test should be made as easy and
+as little complicated for the examiner as possible.
+
+
+III. _Persons to be excluded._--We wish to exclude all those
+individuals who are included in the following three classes: (1)
+Those who see three or less colours in the spectrum. (2) Those who,
+whilst being able to perceive a greater number of colours than three,
+have the red end of the spectrum shortened to a degree incompatible
+with their recognition of a red light at a distance of two miles. (3)
+Those who are unable to distinguish between the red, green, and white
+lights at the normal distance through defect or insensitiveness of the
+cerebro-retinal apparatus when the image on the retina is diminished in
+size.
+
+I will now explain why these three classes of persons should be
+excluded. The first class includes the trichromic, the dichromic, and
+the totally colour-blind, in accordance with the facts previously
+stated. The trichromic never, in ordinary circumstances, mistake green
+for red, but confuse yellow with green or red. Colour is a feeble
+quality of objects to them, and nervousness or excitement may reduce
+them to the condition of the dichromic. The dichromic are liable to
+mistake a green light for red, and vice versa. It is very important
+that persons belonging to the second class should be excluded, and
+yet none of the ordinarily used tests detect them. The rays of red
+at the extreme left of the spectrum are the most penetrating, as may
+be seen by looking at a light or the sun on a foggy day, or through
+several thicknesses of neutral glass. It is chiefly by these rays that
+we recognise a red light at a distance; and it is therefore of great
+importance that a sailor or engine-driver should be able to perceive
+them. The third class contains persons who are able to distinguish
+colours easily when they are close to, but fail to distinguish them at
+a distance, owing to the nerve-fibres supplying the central portion
+of the retina being impaired. As a light at a distance occupies
+the central portion of the visual field, it is essential that the
+corresponding portion of the retina should be normal. There are cases
+of central scotoma for colours with perfect form-vision; these would,
+therefore, not be detected by a test for visual acuity. This class also
+includes those who without having a scotoma are unable to distinguish
+between colours at the normal distance when the image on the retina is
+diminished in size.
+
+
+IV. _The construction of a test for colour-blindness._--In
+the construction of a test for colour-blindness, the facts of
+colour-blindness must be utilised so that the object and requirements
+of the test are fulfilled. The following facts are of practical
+importance.
+
+1. _Most colour-blind make mistakes with certain colours, but are
+correct with regard to others._ This may be illustrated in the
+following way. Let us take an ordinary dichromic, and, having given
+him the set of wools belonging to the Classification Test, ask him to
+pick out all the reds. On examining the pile of wools selected as reds,
+it will be found that the majority are red, but in addition there will
+be some browns and yellow-greens. If he be then told to pick out the
+whole of the greens the greater number of those selected will be green,
+but there will be also greys, browns, and reds. In each case, it will
+be seen that the majority of wools are of the desired colour.
+
+If another dichromic be examined in the same way it will be found
+that, though he may not make exactly the same mistakes, he will in all
+probability pick out the same greens to put with the reds, and the same
+reds to put with the greens. The same result will be obtained if the
+colour-blind persons be asked to name a large number of colours. They
+will in most cases name the colour correctly. It will be noticed that
+the greens which were put with the reds when classifying the colours,
+will be called red in naming them. It is evident that the same idea has
+guided the colour-blind in each case. This shows that, though a person
+may be red-green blind, he is not absolutely red-green blind in the
+sense of being totally unable to distinguish between the two colours.
+The fact that they are actually judging by colour may be demonstrated
+by giving them coloured materials of different kinds, or by asking them
+to name a large number of coloured objects.
+
+It will be seen that if we take a dichromic and ask him to name a
+number of red and green wools, in the majority of instances he will
+name them correctly. But as, almost invariably, the same wools are
+chosen, for all practical purposes the same result would be obtained
+by asking a person to name a few of these wools. What more decided and
+brighter greens could we have than Nos. 76 and 94 of my Pocket Test?
+yet these are two of the greens which are called reds by the dichromic.
+We should have accomplished as much by asking a colour-blind person to
+name Nos. 76 and 94 as if we had asked him to name a large number of
+greens. The colours in a test should, therefore, be those which the
+colour-blind are particularly liable to miscall. At the same time,
+their nature should be unmistakable to the normal-sighted.
+
+2. _The colour-blind name colours in accordance with their
+colour-perception, and thus show definitely to which class they
+belong._ I have not come across a man who has guessed correctly when
+examined with my test. A man who did guess would know that he was
+incompetent. As the colour-blind are often not aware of their defect
+they answer as they see, only guessing when they feel uncertain as to
+the nature of the colour shown. There is probably more misapprehension
+on this point than on any other in the practical testing of
+colour-blindness.
+
+3. _Colours may be changed to the colour-blind, whilst leaving them
+unaltered to the normal-sighted._
+
+4. _The phenomena of simultaneous and successive contrast are much more
+marked for the colour-blind than for the normal-sighted._ Two colours,
+which have not changed in the slightest degree to the normal-sighted
+on being contrasted, have apparently altered very considerably to the
+colour-blind. As an example of this, let us take a pure deep yellow,
+a bright red, and a bright green. To the normal-sighted the yellow
+will be altered very little by comparison with the red or the green,
+but a trichromic would say that the colour was green when contrasted
+with the red, red when contrasted with the green. This principle of
+exaggerated contrast must be borne in mind when examining a candidate.
+Thus if a trichromic be doubtful about a yellow, but seems inclined to
+call it green, he should be given a pure green to compare with it. In
+the same way, in showing the coloured lights, the same colour produced
+in a different way should often be shown. Thus an orange-red may be
+shown immediately after a pure red. This will not alter the colour
+to the normal-sighted, but greatly facilitate the examination of the
+colour-blind.
+
+5. _Many colour-blind match correctly, but name the principal colours
+wrongly._ Therefore the test must be a naming test, the examinee being
+rejected if he confuse the colours which it is essential he should
+distinguish between in his occupation.
+
+6. _Many colour-blind recognise colours easily when they are close to
+them, or the surface is large, but fail to distinguish between them
+when they are at a distance or the image on the retina is small._ The
+test must be constructed in conformity with these facts.
+
+7. _The colour-blind are more dependent upon luminosity than the
+normal-sighted, and are liable to mistake a change in luminosity for a
+change of colour._ The test should have a means of rapidly changing the
+luminosity of a colour.
+
+8. _The colour-blind find special difficulty with faint and dim
+colours._ The test should have colours of this kind.
+
+9. _The colour-blind who have shortening of the red end of the spectrum
+cannot see reds reflecting or transmitting only rays corresponding to
+the shortened portion._ It is essential that reds of this kind should
+form part of the test.
+
+10. _The colour-blind find more difficulty in comparing colours when
+different materials are used, than when the coloured objects are all of
+the same nature._
+
+11. _Most colour-blind find more difficulty with transmitted than with
+reflected light._
+
+12. _The colour-blind have a defective memory for colours._
+
+13. _Colours may be changed to the normal-sighted whilst leaving
+them unchanged to the colour-blind._ When three colours of the
+normal-sighted are included in one of the colour-blind, it is obvious
+that a change from one colour to another of the three will make no
+difference to the colour-blind. Also when the spectrum is shortened,
+the addition or rays corresponding to the shortened portion to another
+colour will not alter its appearance to the person with the shortened
+spectrum. For instance, to a person with shortening of the red end of
+the spectrum, a blue will still remain blue, when so many red rays from
+the shortened portion have been added to it as to make it appear rose
+to the normal-sighted.
+
+14. _The colour-blind may have a sense of luminosity similar to that of
+the normal-sighted._
+
+15. _The dichromic distinguish between the colours of the
+normal-sighted, which are included in one of theirs by their relative
+luminosity and the difference of saturation which is apparent to
+them._ A test should therefore have the means of presenting colours of
+different saturation in succession.
+
+16. _Colour-blindness is frequently associated with very high
+intelligence and exceptional ability._
+
+
+V. _The Lantern Test._[11]--1. _Description of apparatus._ The lantern
+contains four discs: three carrying seven coloured glasses, and one
+with seven modifying glasses. Each disc has a clear aperture. The
+other mechanical details are: an electric or oil lamp with projecting
+accessories, a diaphragm for diminishing the size of the light
+projected, handles for moving the discs and the indicator showing the
+colour or modifier in use. The diaphragm is graduated in respect to
+three apertures to represent a 5-1/2-inch railway signal bullseye at
+600, 800, and 1000 yards respectively when the test is made at 20 ft.
+The glasses are as follows:---
+
+_Coloured glasses._
+
+  1. Red (A and B).
+  2. Yellow.
+  3. Green.
+  4. Signal Green.
+  5. Blue.
+  6. Purple.
+
+_Modifying glasses._
+
+  7. Ground glass.
+  8. Ribbed glass.
+  9. Neutral (No.  I).
+ 10.    "    ( "  II).
+ 11.    "    ( " III).
+ 12.    "    ( "  IV).
+ 13.    "    ( "   V).
+
+[11] Made by Reiner and Keeler, 9, Vere Street, W.; and Meyrowitz, 1a,
+Old Bond Street, W.
+
+[Illustration: Fig. 3.]
+
+It will be noticed that three of the discs are similar in every
+respect. In some of my lanterns the two reds are put at the end of the
+series of colours and numbered Red 1 and Red 2. This makes no important
+difference, but the arrangement given here is more convenient. It
+should be noted that Red 1 corresponds to Red B and Red 2 to Red A. If
+the electric lamp should get broken the projecting apparatus can be
+removed and an ordinary kerosene lamp placed behind the aperture.
+
+[Illustration: Fig. 4.]
+
+2. _Reasons for special construction._--The lantern has been
+constructed conformably with the requirements and facts of
+colour-blindness. All the facts I have given have been considered in
+constructing the lantern.
+
+The examiner, on possessing a lantern for the first time, should
+carefully test himself with it and ascertain how the different lights
+appear to him with different conditions of general illumination.
+It is probable that certain improvements may suggest themselves to
+him, therefore, I think it will be advisable to deal with certain of
+these points, as it will help the examiner in the use of the lantern.
+The colours have never been altered, and I certainly should have
+altered them if I could have improved the lantern by doing so. I
+have never met a single colour-blind person who has not been readily
+and easily detected with my lantern, though I have examined many who
+have passed other lanterns and in some cases a number of other tests
+for colour-blindness. In most cases one turn of the wheel will be
+sufficient to make a colour-blind person disclose his defect.
+
+The examiner may be dissatisfied with the colour of the blue; let us,
+therefore, compare an examination of a normal-sighted person with that
+of an ordinary dichromic. The normal-sighted person will name every
+one of the colours with ease and certainty, with perhaps the exception
+of the blue, with which he is in some doubt. Here is the result of an
+examination of an ordinary dichromic: he called the yellow, green; the
+green, red; the signal green, no colour; the blue, blue; the purple,
+green; red A, no colour or light; red B, green. It will be noticed that
+the only colour that he has correctly named is the blue. We can try
+him again and again, and though he will mistake all the other colours
+he will always name the blue correctly. The examiner will have learnt
+from this several important facts. He will see that the colour-blind
+are really guided by their sensations of colour, and that it is not
+simply a matter of guessing. The more an examiner has practical
+experience of colour-blindness, the more will he recognise the fact
+that the colour-blind are guided by their sensations of colour. He
+will notice that the dichromic has readily recognised the blue which
+was scarcely apparent to him (the examiner), and therefore cannot have
+overlooked as a matter of carelessness colours which are much more
+apparent to the normal-sighted. The blue is a valuable colour for other
+reasons, for though it is not a colour on which I reject candidates,
+anyone miscalling it must be very carefully examined before he is
+passed. The trichromic generally call this blue, green. If we wish to
+obtain a purer blue, we can do so by combining the blue or the purple
+with the signal green.
+
+Again, the examiner may think that it might be better to have an
+apparatus which showed two or three lights instead of only one. I
+will therefore give my reasons for adopting only one. This point was
+one which occupied my attention for a considerable time, especially
+in view of the fact--which, as far as I am aware, I was the first to
+discover--that simultaneous contrast is increased to the colour-blind.
+I was naturally anxious to turn this fact to account. I found,
+however, that I gained nothing by increasing the number of lights, and
+that in many cases it was a source of error. A second or third light
+could have been easily added to my lantern, but besides unnecessarily
+complicating the apparatus it would have served no useful purpose. All
+the results which are obtained with simultaneous contrast are obtained
+even more effectively with successive contrast. It will be noticed
+that when lights are seen in ordinary conditions they are conditions
+of successive contrast, and not of simultaneous contrast. An observer
+rarely keeps his eyes definitely fixed on one light whilst he names
+those adjacent to it, but moves his eyes so that the images of the
+respective lights fall successively on his foveas. When more than one
+light is employed, all the disadvantages of matching as against naming
+are introduced. It will be seen that by presenting one light after
+another we are fulfilling all the necessary conditions, only that the
+light is moved instead of the eyes, sufficient time being allowed to
+elapse to enable a normal-sighted person to readily recognise the true
+colour of the next light without being confused by the after-image of
+the one he has just seen. Many nervous normal-sighted would name a
+yellow light seen between two red lights as green, and it does look
+green to them from ordinary physiological conditions. They look first
+at one red light, then immediately at the yellow light, then at the
+second red light, and then again at the yellow light until they feel
+sure that the centre light is a green light, and say so. I have never
+met with a normal-sighted person who has miscalled the unmodified light
+of my lantern, either red or green. Many humble, nervous normal-sighted
+persons are under the impression that they are colour-blind, and yet
+would make perfectly efficient officers. Many of these men have been
+told by their wives or other persons that they are colour-blind, and,
+believing this, try to see colours which are not visible to them. I
+have examined many persons of this description, and have noted the
+ease and accuracy with which they have gone through the tests for
+colour-blindness when they have been assured by me that they were
+normal-sighted. On the other hand, it is often very difficult to
+convince a self-reliant, colour-blind person that he is colour-blind.
+He is on the look-out for the small differences which he notices
+between colours, and the fact of having another light for comparison
+gives him the desired clue, and, though colour-blind, he passes the
+test.
+
+The material is the best possible, as it will not fade like all dyed
+substances, and therefore all records made with one set of apparatus
+will be uniform. Again, a coloured light has none of the accessory
+qualities which enable the colour-blind to pass through other tests.
+Thus many dichromics will call the yellow glass red or green, who would
+not think of putting a yellow with a green or red wool, on account of
+the difference in luminosity. He will, in the same way, if told to pick
+out colours in the Classification Test to match the colour of the light
+shown, have to depend upon his colour-perception. This is a useful
+method with nervous and undecided candidates. The objection to it is
+that it cannot be carried out in the dark or in a dark room. The Test
+is not open to any of the objections which may be urged against the
+method of simply naming colours, because the character and intensity of
+the colour may be changed at will.
+
+The method is better than that of direct comparison, because the
+candidate is forced to use his colour-perception, and has to compare
+the colour seen with previous impressions of colour in his mind. By the
+use of neutral glasses, etc., I have obviated the fallacy of the method
+of naming colours (namely, that these can be distinguished by their
+saturation and luminosity), and forced the individual to depend upon
+his colour-perception, and not upon some other accessory quality of the
+object seen.
+
+No amount of coaching will enable a colour-blind person to pass this
+test, whilst almost any other may be passed in this way. I have tried
+on many occasions to coach a man so as to pass my lantern, and without
+success. The combinations are so numerous that the only result is to
+make the colour-blind man nervous and doubtful and more easily detected
+than before. This has occurred with men who could pass other tests with
+ease.
+
+The test also has a quality possessed by no other--namely, that of
+enabling the examiner to reject dangerous persons and dangerous persons
+only, the lower degrees of colour-blindness being allowed to pass.
+
+3. _Special directions for conducting the test._--(1) The candidate
+should be seated at a distance of twenty feet from the lantern. (2) He
+should be asked to name the colour of the light produced by a coloured
+glass (1 to 6) alone, or in combination with another coloured glass
+or glasses, or with the modifying glasses (7 to 13). (3) A candidate
+should be rejected (i) if he call the red, green, or the green,
+red, in any circumstances; (ii) if he call the white light, in any
+circumstances, red or green, or vice versa; (iii) if he call the red,
+green, or white lights, black, in any circumstances. (4) A candidate
+who makes mistakes, other than those mentioned above, should be put
+through a very searching examination. It is not necessary to have the
+room absolutely dark; in fact, I prefer a certain amount of light. The
+examiner can, if he wish, make the test at night in the open air.
+
+The examiner should on no account conduct the examination on any
+regular plan, because the candidate, anxious to pass, finds out
+from persons who have already passed the order and method of the
+examination, and so, though colour-blind, might obtain a certificate.
+Any one of the glasses may be shown first, and the candidate required
+to name the colour of the light. The following will serve as an example
+of the method to be employed in testing a candidate. A red being shown,
+the candidate is required to name its colour. Then a blue or green may
+be substituted. It is best to use the largest aperture at first and
+to show all the colours on one disc. This will give confidence to
+the normal-sighted candidate, whilst most of the colour-blind will be
+detected. In the case of candidates who appear to be normal-sighted
+and yet very nervous, there is no harm in telling them after they have
+named all the colours on the disc correctly that this is the case.
+No comment should, however, be made on individual answers. Then one
+of the neutral, ground, or ribbed glasses should be inserted, not
+the slightest intimation being given to the candidate of the nature
+of the colour. He should be asked to name or describe the light, and
+the answer, if incorrect, together with his other replies, carefully
+recorded. The other glasses may then be shown, a combination of the
+neutral, ground, ribbed, and coloured glasses being used at irregular
+intervals.
+
+When the candidate has been examined with the largest aperture,
+the examiner can go through the same procedure with one of the
+smaller apertures. I have found the third aperture the one which is
+most generally useful. On account of the great diminution of total
+luminosity caused by the diminished area of the light source, the three
+smallest apertures can only be used in a dark room.
+
+If a candidate hesitate about a colour and ultimately name it
+correctly, a second and, if necessary, a third glass of the same
+colour should be combined with the first. The fact that in one case a
+single glass is used, and in another two or three of the same coloured
+glass, makes very little difference in the colour of the light to
+the normal-sighted. This is not the case with the colour-blind; a
+dichromic who has hesitated about a green and then correctly named it
+may emphatically call the light red when another green glass is put in
+front of the first.
+
+Care must be taken when the candidate is going to be examined with two
+glasses at once, such as one of the neutral, ground, or ribbed glasses,
+and a coloured glass, that he does not see the light until both are in
+position, or else he may see the colour before it is modified in the
+necessary way.
+
+If the candidate call the standard red, green; or the standard green,
+red, in any circumstances--that is, either alone or in combination with
+the modifying glasses--he is to be rejected.
+
+The examiner should ascertain for himself how far the various colours
+are visible when modified with the neutral glasses. If the red and
+green be not visible with the thickest neutral in the conditions of
+luminosity and external lighting which the examiner is employing, he
+should use the darkest neutral which allows the colours to be plainly
+visible to the normal-sighted. In all cases of doubt the examinee
+should be asked to walk towards the lantern and told to say when the
+light is visible, and asked to name its colour. The distance at which
+the light is visible, and then that at which the colour is visible,
+should be noted and compared with the normal.
+
+Particular attention should be paid to the answers given to the
+combination of the thickest neutral glass with the standard red and
+green respectively.
+
+The examiner should utilise the fact that successive contrast is
+increased in the colour-blind, as this is an easy method of detecting
+the trichromics. The red having been shown, the light should be quickly
+changed to yellow or clear, the examiner’s hand being placed over
+the aperture if there be any intervening colours. It is necessary
+that the yellow should be shown immediately after the red without
+any intervening colours being first seen by the candidate. The
+normal-sighted do not see any change in the yellow or clear when they
+are shown after the red light, but the trichromic call the yellow
+light, green. The examinee should then be shown the green light, and
+then the yellow or clear, in the same way as mentioned for the red. The
+normal-sighted will easily recognise the yellow, but the trichromic
+will call it red. This portion of the examination must never be omitted
+in any examination in which the candidate is passed. The two divisions
+of the test--that is, showing yellow immediately after red and after
+green--may be used at different periods of the examination, and, if
+there be any doubt, repeated.
+
+An examiner should, as far as possible, with the exceptions given in
+the instructions, avoid all conversation with the candidate, simply
+asking, “What colour is this?” and recording the answer without
+comment. If an examiner after each answer say, “Quite right,” or some
+such expression, the following is likely to occur. The candidate after,
+say, six correct answers, makes a mistake; the examiner says, “Are you
+sure?” Then the candidate knows at once that he has made a mistake,
+and makes a guess, very probably a correct one. When a similar colour
+is shown subsequently, he remembers the mistake he made, and gives the
+second, and probably the correct answer.
+
+In addition to being an efficient test, it is a very rapid test, as
+many men who have been certified as normal after a lengthy examination
+with other tests have at once disclosed their defect by calling the
+green light of the lantern red. Many are under the impression that in
+an examination with the lantern the dichromics simply guess. This is
+entirely wrong. A man who did guess would know that he was incompetent.
+I find that men have named the coloured lights in strict accordance
+with their colour-perception. A man may, however, guess if examined
+by an inexperienced and ignorant examiner, who when the examinee has
+made a mistake promptly corrects it in a cross tone. A normal-sighted
+person will guess when examined in this way. The examiner must receive
+the examinee with a smiling face and courteous manner, and appear
+pleased and satisfied with the answers, no matter what they may be.
+The candidate is then placed at his ease, and answers according to
+his colour-perception. It will be noticed that the lantern detects
+those who have a slightly diminished colour-perception, as well as
+the dangerous varieties of colour-blindness. The former undoubtedly
+are not as efficient as those who have a normal colour-perception,
+so that a definite standard will have to be fixed, as in the case
+of visual acuity. Further details will be found in my book on
+Colour-Blindness.[12]
+
+[12] _International Scientific Series._ Kegan Paul & Co., 1909.
+
+_Summary of method of examination._--(1) Show all the colours on one
+disc with the largest aperture. (2) Show the reds, greens, and yellow
+modified by the neutral glasses. (3) Show all the colours on one disc
+with Number 3 aperture. (4) Show red, then immediately afterwards
+yellow with largest aperture. Then show green and yellow immediately
+afterwards. (5) Test the candidate with the red, green, and yellow with
+the smallest aperture. (6) Show the neutrals or ground glass alone. (7)
+Show blue made by combining blue or purple with the signal green. (8)
+Show a colour, for instance, green, and then combine another glass of
+the same colour. (9) Show the red produced by the combination of purple
+with red A. (10) Give the combination of red A and signal green.
+
+
+VI. _Other tests for colour-blindness._--I have three other tests for
+colour-blindness: the Classification Test, the Pocket Test, and the
+Colour-perception Spectrometer. I have also devised an instrument for
+estimating the exact amount of red, at different wave-lengths, which is
+necessary to neutralise the complementary in different persons.
+
+1. _The Classification Test._--(_a_) _Description._--This test
+consists of 4 test colours and 180 confusion colours; 150 coloured
+wools, 10 skeins of silk, 10 small squares of coloured cardboard, and
+10 small squares of coloured glass. The whole series of colours is
+represented. In addition, there are a large number of colours which
+have been chosen by colour-blind persons as matching the test colours.
+The test colours are Orange, Violet, Red, and Blue-green, labelled I,
+II, III, and IV respectively. The colours are chosen with the view
+of presenting as much difficulty as possible to the colour-blind,
+and as little as possible to the normal-sighted. The colour-blind
+find especial difficulty in matching or naming a colour lying at the
+junction of two of their colours. As the normal-sighted often find
+difficulty in saying which colour predominates in a blue-green, so do
+the tetrachromic with their purple-green, or the trichromic with their
+red-green. A colour-blind person may, however, match a colour correctly
+which corresponds to the centre of one of his colours. In addition to
+choosing those colours for tests which are particularly liable to be
+mistaken for other colours by the colour-blind, I have used coloured
+materials of different kinds--wools, silks, glass, and cards--so as to
+force the colour-blind to judge by colour, and not by saturation or
+luminosity. (See Fig. 5.)
+
+[Illustration: Fig. 5.]
+
+(_b_) _Method of examination._--The candidate should be given the four
+test colours, and, having named each, he should be told to select all
+those which are similar in colour to the test colour. He should be told
+to pay no attention to the fact of a colour being lighter or darker; as
+long as it is the same colour it should be put with the test skein. The
+examiner should not go through the test before the candidate first of
+all, neither should one candidate be allowed to watch another making
+his selection. A shrewd colour-blind person might pass the test if he
+had seen a normal-sighted person go through it previously. In order to
+show the candidate the difference between a shade and a colour, the
+examiner should take one of the wools which is not a test colour--blue,
+for instance--and pick out four or five shades of the colour. The wools
+should be arranged without the knowledge of the candidate, so that a
+yellow or a grey is placed beside a red and the examinee asked to name
+its colour. At another period of the examination the yellow should be
+placed adjacent to a green, and the examinee again asked to name it.
+
+The examinee may pick out a certain number of colours correctly, and
+then stop, saying that there are no more exactly like the test colour.
+This may embarrass the examiner; he should, however, examine any
+candidate who has omitted any colours as carefully as if mistakes had
+been made. He should ask the candidate to match one of the omitted
+colours.
+
+The examiner will soon find out from experience those colours which are
+named and matched wrongly by colour-blind persons; he should ask the
+examinee to name some of these colours.
+
+Any candidate should be rejected who calls an orange or red, green
+or brown; black, red or vice versa; or green, either purple, rose,
+red, grey, brown, or violet. Similar mistakes in matching necessitate
+objection. A candidate who puts purple, rose, or blue with violet, or
+yellow-brown with orange is most probably dangerously colour-blind
+and should be very carefully examined. There are cases which pass the
+Holmgren test with ease that fail in the most conclusive manner with my
+Classification Test. They put green with orange, brown and black with
+red, and grey with blue-green. This is due to a different selection
+both of test colours and confusion colours. Orange is by far the most
+important test colour, and its confusion with green by the dichromics
+is very conclusive. The three other test colours, violet, red, and
+blue-green, represent both ends of the spectrum and the neutral point
+in dichromic cases, and practically these colours are those with which
+most mistakes are made. This test can only be regarded as supplementary
+to the Lantern Test.
+
+2. _The Pocket Test._--This consists of nineteen cards, on nine of
+which are 112 single threads of wool, and 14 pieces of twisted silk,
+similar to those in the Classification Test. These are numbered
+consecutively, with the exception of the first thread of the first four
+cards, and the last thread of the next four cards. The end threads of
+the first four cards, I to IV, form the tests; they are Orange, Violet,
+Red, and Blue-green. There are also cards on which red, orange, green,
+blue, violet and purple, and grey, respectively are to be found. There
+are also two special cards marked “Without Red” and two special cards
+marked “Without Green.” (See Fig. 6.)
+
+[Illustration: Fig. 6.]
+
+Many normal-sighted persons might object to the inclusion of some of
+the colours on the orange card, but this card clearly shows the colours
+which may be taken as a match. Fine distinctions are not wanted. The
+series of colours I have selected and arranged so as to confuse the
+colour-blind and force them to be guided by their colour-perception,
+whilst the quantity of colour is amply sufficient for the
+normal-sighted to pick out the colours with the greatest ease. The
+cards should be arranged irregularly on a white cloth in a good light.
+The two most important tests colours are the Orange and Violet, Nos. I
+and II. The person examined should be asked to point out the shades of
+colour similar to No. I (Orange). A piece of paper rolled to a point
+should be used for this purpose. If he do this correctly, he probably
+possesses normal colour-perception. If, however, he match the test
+with reds or pinks, he is more or less colour-blind, at best belonging
+to the pentachromic class. If, in addition, he match the Violet test,
+No. II, with blue, he at least belongs to the tetrachromic class. The
+trichromic, in addition, may match the Blue-green test, No. III, with
+brown and grey. The dichromic will match the Orange test, No. I, with
+yellow-green and yellow-brown. Similar mistakes will be made to those
+described in connection with the Classification Test. The examinee
+should be asked to name all the colours on one of the cards. He should
+also be asked to point out on which of the cards the four test colours
+are to be found, and which contain none of the test colour.
+
+The examiner should continually change the order of the cards. Most of
+the varieties of the colour-blind will be readily detected in this way.
+
+The special advantages of this test are: (1) The colour-blind can be
+ranged definitely in their proper classes. (2) Central scotoma can be
+detected with its aid. (3) The series of colours are arranged so as
+to confuse the colour-blind, whilst the normal-sighted easily match
+the test colours. (4) On account of the introduction of different
+materials, the relative luminosity and saturation of colours does not
+serve as a guide to the colour-blind. (5) Portability. (6) The wools
+and silks are kept clean. (7) An important colour is not likely to be
+lost.
+
+3. _The Colour-Perception Spectrometer._[13]--(_a_) _Description of
+apparatus._--This instrument is a spectrometer so arranged as to
+make it possible to expose to view in the eyepiece the portion of a
+spectrum between any two desired wave-lengths. In the focal plane of
+the telescope are two adjustable shutters with vertical edges; the
+shutters can be moved into the field from right and left respectively,
+each by its own micrometer screw, and to each screw is attached a drum,
+the one being on the right and the other on the left of the telescope.
+On each of these drums is cut a helical slot in which runs an index,
+and the drum is engraved in such a manner that the reading of the index
+gives the position in the spectrum of the corresponding shutter in
+wave-lengths direct. (See Fig. 9.) Thus it will be seen that if, for
+instance, the reading on the left drum-head is 5320 and that on the
+right drum-head is 5920, the region of the spectrum from wave-length
+5320 to wave-length 5920 is exposed to view in the eyepiece.
+
+[13] Made by A. Hilger, 75a, Camden Road, London, N.W.
+
+[Illustration: Fig. 7.]
+
+[Illustration: Fig. 8.]
+
+[Illustration: Fig. 9.]
+
+(_b_) _Directions for using the instrument._--It should be used as
+far as possible with a known quality and intensity of light. A small
+oil-lamp is quite suitable for the purpose. The observer should
+first ascertain the exact position of the termination of the red
+end of the spectrum, the left-hand shutter being moved across until
+every trace of red just disappears. The position of the pointer on
+the left-hand drum is noted, and the wave-length recorded. The left
+drum is then moved so that the shutter is more towards the middle of
+the spectrum. The right-hand drum is then moved, until the pointer
+indicates the wave-length recorded as the termination of the red end
+of the spectrum. The observer then moves the left-hand shutter in
+and out until he obtains the largest portion of red, which appears
+absolutely monochromatic to him, no notice being taken of variations
+in brightness, but only in hue. The position of the index on the
+left-hand drum is recorded. The left-hand shutter is then moved towards
+the violet end of the spectrum, the right-hand shutter being placed at
+the position previously occupied by the left-hand shutter. In this way
+the whole of the spectrum is traversed until the termination of the
+violet end of the spectrum is finally ascertained with the right-hand
+shutter. The variation of the size of the patches and the terminations
+of the spectrum with different intensities of light can be noted. The
+instrument can also be used for ascertaining the exact position and
+size of the neutral patch in dichromics, the position of greatest
+luminosity, and the size and extent of pure colours. When it is used
+to test colour-blindness, the examinee should first be shown some
+portion of the interior of the spectrum, and then asked to name the
+various colours which he sees. In this way he will have no clue to the
+colours which are being shown him.
+
+_Objections to other tests for colour-blindness._--The tests which
+have been proposed for colour-blindness are very numerous, but some
+are so defective that it is rare to detect a single colour-blind
+person with them. I have, for instance, tested men whom I knew to be
+colour-blind with certain lanterns with the result that not a single
+one was detected. In these so-called tests all the requirements of
+a test and facts of colour-blindness have been neglected. I must,
+however, refer to three tests constructed by exceptionally able men,
+each with considerable knowledge of the subject. I refer to the tests
+of Professor Holmgren, Professor Stilling, and Professor Nagel.
+
+All these tests can be passed at the first attempt without coaching by
+certain dangerously colour-blind persons, chiefly varieties not known
+to the inventors, but the chief defect of each is that it is very easy
+to coach a colour-blind person to pass it. The surgeon to one of our
+largest railway companies told me that when they used Holmgren’s test
+they rejected one man in three hundred, but with my lantern twelve in
+the same number. All these three tests are much better tests when the
+persons to be examined have not seen them before. A colour-blind man
+may make only one mistake, say for instance, as in a case I examined
+the other day, with Nagel’s test (last edition), he passes the test
+perfectly with the exception of one mistake, that of calling a grey on
+one card, green. All he has to do is to look for some distinguishing
+mark on this card in order to go through the test with the ease and
+certainty of a normal-sighted person. It is the same with Stilling’s
+letters, he has only to note the letter which he was not able to read
+and the appearance of the card. A normal-sighted man or woman would
+readily help him. The confusion of green and grey does not appeal
+to the average man as a serious defect, especially when he sees his
+friend go through the rest of the test perfectly. He says to himself,
+“I suppose he sees a tinge of green in that grey.” The same man would
+rightly regard it as a most iniquitous proceeding to endeavour to coach
+his friend through a test when he had seen him mistake a red for a
+green light.
+
+Holmgren’s test rejects a large number of normal-sighted persons, as
+may be seen by the reports of the Board of Trade; about 50 per cent
+of those who appeal are found to be normal-sighted and to have been
+rejected wrongly.
+
+
+  WILLIAM BRENDON AND SON, LTD.
+  PRINTERS, PLYMOUTH
+
+
+Reiner and Keeler, L^{td.}
+
+  OPTICIANS
+  ----AND----
+  INSTRUMENT MAKERS
+
+
+_MANUFACTURERS OF_
+
+  The Edridge-Green Colour Perception Lantern
+  The Edridge-Green Classification Test
+  The Edridge-Green Pocket Test
+  and other
+  Optical and Scientific Instruments
+
+
+THE ABOVE COLOUR TESTS ARE CERTIFIED BY PROF. F. W. EDRIDGE-GREEN
+
+  9 Vere Street,
+  Cavendish Square, =London, W.=
+  Telephone: 447 Mayfair.
+
+
+
+*** END OF THE PROJECT GUTENBERG EBOOK 75519 ***