CHAPTER XXI.

OBJECTIONS TO THE TESTS FOR COLOUR-BLINDNESS IN GENERAL USE.

1. An Examination with Coloured Lights.-The first idea which would naturally occur to any one wishing to test persons, who would in their employment have to distinguish between the red, green, and white (yellow) lights, would be to test them with these lights as occurring in their employment: for instance, in testing a candidate for employment as engine-driver, to take him at night to some junction where a large number of signal lights could be seen, and ask him to name the colour of the lights as they were pointed out to him. What, therefore, are the objections to testing in this way?

In the first place we can not obtain at will the varying atmospheric conditions which a man would meet with in his employment. Thus fog, rain, hail, and snow, each produce a different aspect of the light. So that even if we could wait until we obtained each of these conditions (which of course would be quite impracticable), we could not obtain the rapidly changing atmospheric conditions with which an engine-driver meets. He may have fine weather in one part of his journey, rain in another, and fog at the terminus. Again, a man, when he is standing still looking at a number of red and green lights, can compare the two, and so make up his mind which is the

red and which is the green. When running his train, he sees only one light, and has to decide quickly whether it is red or green.

As a matter of fact, colour-blind persons can, as a general rule, distinguish between the red and green lights, and so remain in the employment of a railway company for years without causing an accident, but one may occur at any moment through their physical defect.

I have taken a completely red-green colour-blind person to a place from which a number of signal lights were visible, and asked him to give me the name of each. This he did easily and without any mistakes.

2. Objections to Holmgren's Test. This test is one which is well known, and has many advocates. It is one that has formed the basis for many others. Before proceeding to criticize his test, I must again offer my tribute to the value of Professor Holmgren's services in bringing the subject of colour-blindness forward, and regret that I have to condemn his test. This test is based on one of the most popular theories of colour-vision, and in practice results come out in many respects as Holmgren says they will. Let us consider it under the following heads :

1. Theory. I have previously dealt with the objections to the Young-Helmholtz theory of colour-perception on which this test is based. In that chapter I have dealt with Holmgren's test in so far as it relates to the theory. It repeats all the errors of the theory upon which it is based.

2. Material. The material for the purposes of a colourtest is not a good one. First, we have the difficulty of obtaining correctly coloured wools; then there is the great difficulty in obtaining duplicates of these wools. As I have shown in the chapter on the composition of colours

because two colours look alike it is not by any means necessary that they have the same composition. The probability is that if the dyes used are different the colours will differ in composition. As a matter of fact the sets of wools which are sold in this country bear a very feeble resemblance to Holmgren's true set.

When a good set of wools has been obtained, the colours are liable to fade; and this fading will take place more rapidly in the case of some colours than in others. This will prevent the results of examinations made with one set of wools from being uniform and comparable.

Then, the wools will become soiled, and this will happen in a very short time if the testing be conducted chiefly amongst the lower orders.

Then the greens which it is necessary to pick out will become more soiled than the others, and so afford a mark of distinction to the colour-blind, who, not being confused by the striking differences of colour, are able to notice minutiae of this kind. It will be found, in testing a large number of persons, that those who are colour-blind will try to find some distinguishing mark in the wools selected by others.

There are other points connected with the material which enable the colour-blind to pass the test. Holmgren himself relates a case of a man who could distinguish between brown and green by the touch, but not by the colour.

The relative luminosity of the colours of wools is very great, and forms a distinguishing point to the colourblind. An ordinary red-green colour-blind will not put a yellow wool with a green or red one, but he will put yellow glass with green or red glass.

3. Method of Testing. With regard to the method which is employed in this test, Holmgren says:—

"Theoretically our method most resembles those of Seebeck and Maxwell, as it is based upon a comparison between different colours. It therefore seeks to discover the chromatic perception of the subject, disregarding the names he gives to the colours, as generally it is not necessary he should designate the names."

It will be seen that one great difference between Holmgren's method, and mine in the use of the Classification Test, is that he says that the names given to the colours may be disregarded, whilst I am of opinion that a person cannot be efficiently tested if this be done.

The method of matching colours should, to be properly carried out, be one of mentally naming them, or else the test is useless on account of this point alone. For instance, if a man say to himself, "This test colour is green; therefore I must pick out all the colours having this hue of green in them," he will go through the test as it should be gone through; but if, on looking at the wool, he be influenced more by shade, he will put light blues, yellows, grays, and browns with the light green. This will be especially liable to happen in cases of the lesser degrees of colour-blindness, in which the green is simply enlarged and encroaches on the yellow and blue.

When using Holmgren's test in strict accordance with his instructions, I have examined normal-sighted persons who have put grays and browns with the test green. After a short lesson in colours, these persons were able to classify a large number of coloured objects correctly, and explained that the browns and grays were very like the test green in shade. Similarity, other than that defined by the word "colour," is the great source of error in this test, and it is exaggerated by ignoring colour names.

I am informed that applicants for situations on the railway who have failed with Holmgren's test, have gone

to a Berlin wool shop and been instructed by one of the saleswomen. After this they have again gone up for the test, and passed easily. Comment is unnecessary.

The question then comes up for consideration: What are the differences between matching and naming colours? When a person is asked to name a colour without being allowed to compare it with other colours, he has to compare the present impression of colour with those impressions already in his mind. In order, therefore, that he may be able to name colours, he must at least have an elementary knowledge of colour-names. A person who is required to match colours may be guided chiefly by colour or chiefly by shade; in fact, by similarity of any kind. A person who has been educated with regard to colour will be chiefly influenced by colour; but a man who has only an elementary knowledge of colours will be influenced considerably by shade. If we compare the two methods with persons belonging to the educated classes, we shall see how inferior the method of matching is to that of naming. Let us take a hundred and fifty wools, and examine two similarly colour-blind educated persons with them. The first is asked to classify the wools, putting all those of similar colour together, the second is asked to name the whole of the wools one by one. It will be found that the second person will make very much more numerous mistakes than the first. The reason of this is obvious: in the case of naming colours, the person has only the impression present in his mind to enable him to name a colour correctly; in the case of matching colours, he has other colours present with which he can compare the test. We all know what to do when we are doubtful what to call a colour-namely, compare it with some colour about which there is no doubt. Slight differences of colour become more marked on comparison.