Colour vision

by Sir William de Wiveleslie Abney

CHAPTER XIII.

You have been taken through much experimental work, and possibly it may be thought that there has been too much of it; but now that we are coming to the more practical part of the subject, it will become apparent that a good working hypothesis is absolutely necessary before effectual tests for colour vision can be carried out, and that the reasons for its adoption should be given in full. The question of colour blindness is one of very practical importance, as in certain occupations it is essential that colours should be accurately and quickly known, and that no guess-work should be allowed. Lives have without doubt been lost by a want of proper knowledge of colours, both at sea and on railways. The evidence that such is the case is, as a rule, it is true, merely negative, though there are cases extant where great losses which have occurred can be traced to a deficiency in colour perception. If there be no proper system of tests for ascertaining the defects of signal or look-out men in their colour sense, it is palpable that positive evidence cannot be forthcoming, and this is very much the state of things which exists up to the present time. We hear of collisions at sea and vessels foundering in consequence of the rule of the road not being followed, but at the investigations which follow we have no record that the question of colour perception of the look-out man has been gone into, though there may be conflicting evidence as to whether a red light or a green light was shown. That danger from colour blindness is incurred has for some time been recognised by the Board of Trade, as it insists that all officers of the Mercantile Marine must be tested for their sense of colour, and that their certificates must be endorsed as having failed to pass the colour test should they do so. For my own part, I think endorsement of their certificate is quite inadequate, for it is still open for shipowners to employ them (of course at their own risk). A rejection for colour vision should entail a withholding of the certificate altogether; for it surely is as dangerous that a signal should be misread as it is that the logarithm of the sine of an angle should be misunderstood. If a candidate fails in theoretical navigation, he is not allowed a certificate, but if he only fails in a very practical part of his examination, his certificate is merely endorsed.

The system employed by this department was a defective one, and we know of many instances in which candidates have passed the colour test, though they ought to have been rejected, and are at present in the service. The subject of testing for colour vision was brought prominently forward some two or three years ago, and a Committee of the Royal Society, to which I acted as secretary, was requested to consider the methods at that time in force on the railways and in the mercantile marine, and to find one which was not open to objection. It recommended the system that had been elaborated by Holmgren, a Swedish physicist, and known as Holmgren’s test, which has long been in force in Sweden and elsewhere. This system has, I am glad to say, been adopted by the Board of Trade, and by most of the railway companies in the United Kingdom. There have been numerous indications that this change of method was necessary. Only within the last month (April, 1894), for instance, I was informed by the Medical Officer who had to examine the employés on a certain railway in Scotland by the Holmgren test that he had found some, amongst others an engine-driver, who were colour blind, and presumably unfit for the posts they occupied owing to this defect.

There is one popular objection which is always made against this test, or indeed against any proper test, viz., that the examination is not made under the same conditions which absolutely exist, nor with the very lights which the candidates have to distinguish from one another—that is, the red and green lights. Let me beg of you to remark, that as a mere matter of guessing, the chances are equal that a man would name the light shown correctly. If you turn a man’s back to the light, and if he has a coin in his pocket and deliberately calls heads red and tails green, he will have a good chance of passing the test; for, if he guessed rightly three or four times, no one would fail to pass him on his answers. The great point in a test is to cause the candidate to do something to show that he appreciates colour. It is this doing something and saying nothing which is the important feature in the Holmgren test. A man may be ignorant of the names of colours—colour ignorant it is called—but he cannot be ignorant of the colours themselves if he has normal colour vision. As a matter of fact, the colour blind may possibly distinguish between red and green lights by having carefully noted, under ordinary conditions of atmosphere, their different brightness, and by their difference in saturation with their neutral colour. If external conditions are altered, as they are in actual daily life, these slight indications vanish, and the quick naming of the colour to be read becomes a mere matter of chance. A proper test should include all variations that can occur in these respects. It cannot be too strongly impressed upon every one that a man who is colour blind to colours in ordinary daylight is equally so in lamplight, although some shades of colour which are well distinguishable by daylight may disappear when the artificial light is used as the source of illumination.

Now, on what scientific principles should a colour test be founded? We must hark back to a theory for a moment, and as it has been shown that for all essential purposes that of Young answers, we will use it as a good working hypothesis, and it was from this theory that Holmgren himself reasoned. The red- or green-blind see a grey in a part of their spectrum, which to us who possess normal colour vision is green. If then we present such a green to them, they would match it with a grey. If, however, we have a yellowish-green, which is pure green mixed with red, the complete green-blind will not see the green in it, but only the red. The colour to him would be very pale red, and as he sees all such greens and yellows and reds as red more or less saturated, that is, more or less mixed with his neutral colour, any one of these he would match with a green. The red-blind, on the other hand, would see all these colours as green, and he too might make similar matches with them. Suppose now we have a pink skein: the green-blind would see it as a white or bluish-white, for a purple is white to him, and he would match with it either greys or colours having a slight excess of blue in them; for a green is to him a neutral colour. The red-blind, on the other hand, would see but little green in the pink; blue would predominate, so he would choose mauves or blues amongst other matches.

Acting on these principles, Holmgren selected his test colours. He chose wools as the most convenient for handling, and also because they present the same colour without sheen when looked at in any direction. His first test colour is a very pale green which contained no blue. Its paleness is a point in its favour. The colour is quite distinguishable by us normal-visioned persons, but it might appear as grey to the red- and green-blind; for as we who possess normal vision may mix a small percentage of colour with our neutral colour (white) without it being perceived, so may they with theirs (white and green). As the green, when it is to us saturated, would be nearly neutral coloured to them, the very diluted colour which we see in the skein would to them be masked by the addition of white. In any case, if any colour be visible to them, it must be on the red side of the neutral points. A candidate is given this skein of wool, and from a heap of over a hundred skeins, of varying degrees of saturation, amongst which are drabs, yellows, yellow-greens, blue-greens, purples, pinks, greys, and so on, he is asked to select others which appear to him to be of the same colour as the test-skein, though they may be darker or lighter. He will, if colour blind, select some of the colours already indicated. The second test-skein is a pink, which is a purple diluted with white, but much less so than the green, to which it is nearly a complementary in daylight. The candidate is required to select colours which match this, and according to his selections is he pronounced as having normal colour vision or as being colour defective (either completely or partially) to the red or to the green. The case of violet blindness is not important in reading the signals ordinarily used, and therefore in this test no special test-skein is employed. Let us consider what colour we should use. The neutral colour to this form of colour blindness is yellow. If, therefore, we pick out a pale yellow skein, the candidate would pick out greys to match it; or if we gave him the pink skein to match, since he has no blue (violet) sensation, he would match it with a pure red or with a purple.

Where monochromatic vision is under examination, all skeins would be matched with one another indiscriminately—blues, reds, greens, greys will all be a match, some lighter and some darker than the test-skein. I have been told by some who have carried out examinations for colour blindness that this matching is by no means so uncommon as is often imagined. In future it is hoped that most of those who make these matches may be examined by the spectrum test, as it may turn out that a proportion of them will be most valuable theoretical cases.

In making an examination with the Holmgren test, it is almost unnecessary that the candidate should take up a skein out of the heap of wools to form a preliminary diagnosis. The colour blind will not at once pick out an evident match, but will hesitate and evince a desire to appear very accurate in his choice. This indicates at once that there is something amiss. He probably will pick up a skein of the right colour, place it against the test-skein, lay it down and again take it up. Or he will pick up a skein which is evidently incorrect and do the same thing, but perhaps he will return it to the heap and take up another which is equally bad.

He will fumble over making his matches, and eventually have a heap by him which will at once tell the examiner that he is colour defective. I may as well give you an idea of the colours which the colour blind will pick out by a simple experiment. The heap of wools is on the table, and in the pure white of the electric arc light, which is thrown on it from the lantern, every colour is distinct in hue and in intensity. On one side are placed the two important test-skeins, the pale green and the pink. There can be no doubt but that in that heap of wools there are a large number which can be matched with each of them. The red-blind, be it recollected, sees no red, and if I can place in front of the lens of the lantern some medium which cuts off the red as completely as possible, the audience as well as myself will see the colours approximately as the red-blind would do. Such a medium is found in the same blue-green glass that is used for signals on most railways and on board ship. The green-blind, on the other hand, see no green, and if a medium can be found which when placed in the path of the light allows no green to pass, the colours in the heap being deprived of the green would be such as would very nearly be the same as this type of colour blind would see. This glass is covered with a film of collodion in which fuschin and blue have been dissolved. It transmits a fine purple and should answer our purpose. That these two media are what we require can be readily demonstrated by placing them in front of the slit of the collimator of our colour apparatus and throwing the spectrum on the screen. The spectrum of white light is now on the screen, and when we place the blue-green glass in front of the slit, we see that the red is very nearly entirely extinguished, whilst if we substitute for it the dyed collodionized glass the green is absent. Now, placing the first glass in front of the lantern lens and switching on the current, the wools are illuminated with the bluish-green light. The green test-skein appears green, and we can proceed to make our matches, picking out the colours which appear the same, but taking no heed as to their lightness or darkness. A dozen skeins are now picked out, and I think the audience will agree with me that the matches as viewed in the green light are accurate. The glass is now withdrawn, and the ordinary white light falls upon the skeins in my hand. They are a strangely variegated lot as now seen; we have green shades, yellows, and browns, and greys. Such a variety would tell me that I was colour deficient, but would not be, perhaps, decisive as to what was the exact character of the deficiency. For if the pink glass is placed in front of the lantern you will find the same matches, with one or two exceptions, might have been made. The blue-green glass is once more placed in the beam, and this time I match the pink skein with the wools. A certain number are picked out, and the audience will agree with me that the matches are fair ones. When, however, the glass is withdrawn from the light and we see what colours have been selected, we find that they consist of pale blues, mauves, pinks of various shades, and cerise, and violet. The red in the pink did not affect my eyes any more than would it the red-blind. I am evidently then in this light red-blind, for if the pink glass replaces the blue-green, the matches are impossible. While this coloured light is illuminating the heap I will make matches again. When made, the white light is again thrown on the selected skeins, and this time we have bluish-green and neutral tint together with pinks. The reason of this is evident, there is no green visible; the bluish-green contains besides blue a certain amount of yellow, which, in its turn, contains red, and the grey must be pink. To the green-blind, for reasons already given, the blue-green looks white, as does the pink, and therefore the two are matched together. The grey is also degraded white to him, and therefore he also matches that with them. The matches which the violet-blind would make can be well exemplified by placing in the beam of light a yellow glass, or a glass coated with collodion in which “brilliant yellow” has been dissolved. By this plan, then, we can in some measure produce the effect of colour blindness on ourselves, and very interesting it is to compare theory with the results obtained in this manner. There is no necessity to have recourse to the electric light for this purpose. If matches are made with such media held before the eyes in ordinary daylight, the same results will be obtained. I have often examined through these same media the matches made by the colour blind, and been able at once to settle the nature of the defective vision from which they were suffering. It must be remembered that the colours transmitted through these two glasses are not absolutely like the whites which the two classes of colour blind see respectively, though they approach it.

We can imitate even more exactly the matches that they would make by matching white light with a mixture of red, green, and violet of the proper hues, and covering up the red or green slit, and then placing the test-skein and the matches in the colour so formed. From the other skeins viewed in the same light can be picked out the matches which would be possible. There is very little chance, if any, of a mistake about them being made when this plan is adopted.