Part 3

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._