When observations have been made at numerous places in the spectrum, the tops of these ordinates, as they are called, should be joined, and we then get the observed curve of luminosity for the whole spectrum. For convenience’ sake we make the highest point 100, and reduce the other ordinates in proportion. For some purposes it may be advantageous to give the luminosity curve in terms of a scale of wave-lengths. For our purpose, however, it is in general sufficient to use the scale of the instrument.
Fig. 19.
Now, if we test the vision of the various types of colour blind by this plan, we should expect to get luminosities at different parts of the spectrum which would give very different forms to these curves. We cannot hope, for instance, that a red-blind who sees no red in the extreme end of the spectrum would show any luminosity in that region, nor that the green-blind should show as much in the green part of the spectrum as those who possess normal colour vision, since one of the sensations is absent. With monochromatic vision there should be a still further departure from the normal curve. That these differences do exist is fully shown in [Fig. 19]. One of the most striking experiments in colour vision is to place a bright red patch on the screen, and to ask a red-blind to make a match in luminosity with the white. The latter will have to be reduced to almost darkness—a darkness, indeed, that makes the match almost seem incredible. You will notice that the places in the spectrum where the red- and green-blind see grey are by no means places of greatest luminosity. We shall find that these luminosity curves are suggestive when making another investigation into the form of the spectrum curves of the colour sensations.
Besides cases of complete blindness due to the absence of one or two sensations on the Young theory, we have other cases, as was said when remarking on the percentage of people who are colour deficient, in which one or even two sensations are only more or less deadened. It has often been said that with the theory provisionally adopted, such cases are difficult to class as red or green deficient. As far as my own observations go, I have never found this difficulty. The luminosity curves of such observers, combined with other indications, give a ready means of classing them. The main difficulty to my mind is to state what is normal colour vision, but, as I have found that the very large majority of eyes give the same luminosity to colours as my own, I have taken my own colour perception as normal. In numerous experiments which Lord Rayleigh has made in matching orange by means of a mixture of red and green, he has come across several who have apparently normal vision, as they see colours correctly in every part of the spectrum, and yet some require much less red mixed with the green to make a match with the orange than do others. What is yellow to them is decidedly green to the majority. This has been classed as another kind of normal vision; but the luminosity curves show that it may be equally well due to a deficiency in the green sensation, and which would require more green to make the necessary match. The limits of the visible spectrum to these persons, as far as my examination of their cases goes, are the same as my own.
Again, there are others in which the spectrum seems decidedly somewhat shortened at the red end compared with my own, and the luminosity curves point to them as being strictly colour deficient in the red and nothing else. As they see all colours, they have been classed as another form of normal vision. The deficiency in both these cases is so small that white is their neutral colour, but there is evidence that the hues are slightly changed. I do not wish any one to accept my deductions as being more correct than those who hold differently, but the results of examination by the luminosity methods appear to me difficult to reconcile with any other view. There are, however, a large number of cases in which, though complete red- or green-blindness is wanting, there is no doubt that more than slight colour deficiency exists. For instance, in [Fig. 23] we have the curve of luminosity of the spectrum as measured by a very acute scientific observer, and it is compared with that of normal colour vision. He certainly is not completely blind to any sensation. An inspection and comparison of the two curves will show that he is defective in the green sensation, although it is present to a large extent. The deficiency is obvious enough. An endeavour to find his neutral point was most interesting. At 39 in the scale he saw a little colour, but at 39·5 all colour had vanished, and between the coloured patch and the white he saw no difference. This similarity he saw till 47·3 in the scale, when he began to see a faint trace of colour. There is a large piece of the spectrum, then, which to him is grey. It must be recollected that all three sensations were excited in this region, but some more than others. Now, experiment has shown that, with normal vision, two per cent. of any colour may be mixed with a pure colour without its being perceived. It is not surprising, therefore, that although the red, or the green, or the blue may be present in an intensity above that required to form white, yet the resulting sensation should pass for white. It may be remarked that red and white when mixed he never mistook for yellow, and he always recognised yellows and red; yellowish green, however, he called pale yellow.
Fig. 23.
Another example of partial red-blindness is also instructive. [Fig. 23] also shows it graphically. There is no doubt as to the nature of the defect. The spectrum is slightly shortened, and the luminosity of this part of the spectrum is less than that of normal vision. There was no difficulty in distinguishing every colour, though the positions of the colours from yellow to green seemed to be shifted; but no neutral point could be traced. Apparently, both this case and the former are about equally colour defective; but in this last the same reasons do not apply for the existence of a neutral point. (For measures see [page 214].)