360 R = 56 W + 304 B (corrected),
360 U = 60 W + 300 B (corrected).
With any proportion of R and U mixed together she matched a grey of approximately the same intensity as above, as it might be supposed she would from the last two equations.
Taking the intensity curve of the light reflected from the red disc, it was found to contain a great deal of the part of the spectrum which she called brownish, viz., from 33·5 to 48 on the scale, whereas the blue reflected a trifle of this portion of the spectrum, as did also the green; and this may account for her making a match to grey of U and G, and not of R and G, but it is hard to see why she matched U alone and also R with the grey.
Reviewing the case, it seems that any perception of colour is very small, and that the sensations are green and much less red. From the equations it also seems that she would have matched green with white and black alone, and that 360 G = 75 W + 285 B. Perhaps the explanation of the matches and names of colours may be that a proportion of colour may be mixed with another without being perceived, but this colour so hidden has still the capability of neutralising a certain quantity of the complementary colour.
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.