CHAPTER VI.
Another mode of exhibiting colour blindness, and one of the first adopted, is by making mixtures of colours with rapidly rotating colour discs. In my own experiments I have chosen a red, which is scarlet, over which a wash of carmine has been brushed. It has a dominant wave-length of 6300. The green is an emerald-green, and has a dominant wave-length of 5150. The blue is French ultra-marine, with a dominant wave-length of 4700. The card discs, of some 4 inches diameter, are coated with these colours as pastes, and by making an incision in them radially to the centre, as before described, and inter-locking them, the compound disc can be caused to show sectors of any angle that may be required. Outside these are the discs of black and white, the proportions of which can be altered at will.
The light thrown on the rotating sectors being that from an electric arc light, normal vision requires 118° of red, 146° of green, and 96° of blue to match a grey made up of 75 parts of white and 285 parts of black. For the last two numbers a correction has been made to allow for the small amount of white light reflected from the black surface. This correction has also been made in the subsequent matches which will be described. Colour mixtures such as these are conveniently put in the form of equations, and that given will then be shown as follows—
118 R + 146 G + 96 U = 75 W + 285 B.
(Here R, G, U, W, and B are used to indicate Red, Green, Blue, White, and Black.)
This match was exact also for all the colour blind, for the deficiency in one grey is also a deficiency in the other. With a red-blind, however, very different matches can be made, as the red pigment is a complex colour. There is in it, besides red, a certain amount of yellow, whilst in the green there is, besides green, a small amount of a red and a larger amount of yellow. The yellow will not only stimulate the green sensation, but also the red where it is present. Although in complete red-blindness the red sensation is totally absent, we may expect that a mixture of red and blue, as well as of green and blue, will enable a match to be made of the grey produced by the mixture of white and black.
This was the case. We have the following proportions—
295 R + 65 U = 45 W + 315 B.
When the green disc is substituted for the red, the red-blind made the following mixture—
229 G + 131 U = 120 W + 240 B.
It is worth noticing that the amount of blue in the first mixture is about half that required for the second. This tells us that the amount of green sensation stimulated in the first case is much less than in the second. As red can be substituted for green, it should follow that green, when rendered darker, should match the red. To try this a red disc replaced the black disc, and a black disc replaced the blue. The following match was then made—
131 G + 229 B = 340 R + 20 W.
It seems impossible to believe that these mixtures, so dissimilar in colour, could ever form a satisfactory match. This last equation might have been derived from the two first, in which case it would have stood—
137 G + 223 B = 342 R + 18 W.
By a completely green-blind the following mixtures were made—
251 R + 109 U = 62 W + 298 B,
and
277 G + 83 U = 107 W + 253 B.
In this case 363 Green are equivalent to 251 parts of Red mixed with 78 of White and 34 Black. The difference in the matches made by the two types of colour blindness is very evident. In the one case the amount of red required is much greater than the green, and in the other vice versâ. Another instance may be given of colour matches made, by means of discs, by a partially green-blind person, whose case will be more fully described when we treat of the luminosity of the spectrum to the different classes of colour vision.
His matches were as follows—1st, That of the normal vision. 2nd,—
160 R + 80 G + 120 U = 72 W + 288 B.
The green was then altered to 200, when the following made a match—
65 R + 200 G + 95 U = 72 W + 288 B.
Using these two equations, we have the following curious result—that 120 G was matched by 95 R + 25 U. As the green disc is nearly twice as luminous as the red to normal colour vision, this equation confirms the result otherwise obtained, that his blindness to colour is a deficiency in the green sensation. No mixtures of blue and red, or blue and green, would match a grey formed by the rotation of the black and white sectors.
I must now introduce to your notice a different method of experimenting with colour vision. If we throw the whole spectrum on the screen, and ask a person with normal vision to point out the brightest part, he will indicate the yellow, whilst a red-blind will say the green, and so on. This tells us that the various types of colour blind must see their spectrum colours with luminosity differing from that of the normal eye. The difference can be measured by causing both to express their sense of the brightness of the different parts of the spectrum in terms of white light, or of one another. Brightness and luminosity are here used synonymously. On the two small screens are a red and a green patch of monochromatic light—a look at the green shows that it is much brighter than the red. Rotating sectors, the apertures of which can be opened or closed at pleasure during rotation, are now placed in the path of the green ray. The apertures are made fairly small, and the green is now evidently dimmer than the red. When they are well open the green is once more brighter. Evidently at some time during the closing of the apertures there is one position in which the red and green must be of the same brightness, since the green passes through the stage of being too light to that of being too dark. By gradually diminishing the range of the “too open” to “too closed” apertures we arrive at the aperture where the two colours appear equally bright. The two patches will cease to wink at the operator, if we may use such an unscientific expression, when equality in brightness is established. This operation of equalising luminosities must be carried out quickly and without concentrated thought, for if an observer stops to think, a fancied equality of brightness may exist, which other properly carried out observations will show to be inexact. Now, instead of using two colours, we can throw on a white surface a white patch from the reflected beam, and a patch of the colour coming through the slit alongside and touching it. The white is evidently the brighter, and so the sectors are placed in this beam. The luminosity of (say) a red ray is first measured, and the white is found to require a certain sector aperture to secure a balance in brightness. We then place another spectrum colour in the place of the first, and measure off in degrees the brightness of this colour in terms of white light, and we proceed similarly for the others. Now how are we to prove that the measures for luminosity of the different colours are correct? Let us place three slits in the spectrum, and by altering the aperture of the slits make a mixture of the three rays so as to form white. The intensity of this white we can match with the white of the reflected beam. We can then measure the brightness (luminosity) of the three colours separately, and if our measures are correct there is primâ facie reason to suppose that they will together make up the brightness of the white. Without going through this experiment it may at once be stated that the reasoning is correct, for within the limits of error of observation they do so. Having established this proposition, we can next compare inter se, the brightness of any or all of the rays of the spectrum by a preliminary comparison with the reflected beam of white light. As in the colour patch apparatus all colours and principal dark lines of the solar spectrum are known by reference to a scale, in making a graphic representation of the results, we first of all plot on paper a scale of equal parts, and at the scale number where a reading is made, the aperture of the sectors in degrees is set up. Thus, suppose with red light the scale number which marked the position of the slit was 59, and the aperture 10°, we should set up at that scale number on the paper a height of 10 on any empyric scale. If in the green at scale No. 38 the sectors had to be closed to 7°, we should set up 7 at that number on the scale.
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].)