§ 7. Partial Colour-Blindness.—Between the outer margin of the retina and the yellow spot, there is a region which is partially colourblind. It is sensitive to blue and yellow, but not to red and green. This may be tested by an experiment similar to that described in the previous section. "When the colours of the spectrum are seen sideways, so that they fall on the partially colourblind zone of the retina, the bluegreen region appears grey. This grey divides the whole spectrum into two parts. The part containing light of greater wavelength appears yellow, that containing light of smaller wavelength, appears blue. Red and green are not discernible.
It is well known that there are many persons whose whole retina is affected by a partial colour-blindness, consisting in an inability to distinguish between red and green. Now, abstractly considered, this inability to distinguish between red and green may arise in either of two ways. A person who was insensitive to both red and green could not of course distinguish them from each other. But the same might hold true of a person sensitive to red and not to green, or to green and not to red. If we suppose yellow to be due to a combination of the retinal processes which are produced by red light and green light respectively, persons insensitive to red would see all yellows as green, and those insensitive to green would see all yellows as red. Both modes of explaining partial colour-blindness have been, and still are, advocated. On the whole, it seems most probable that in the partially colourblind the retina is equally incapable of giving rise to sensations either of red or green. But the question is full of difficulty. The evidence shows clearly that there are two distinct types of partial colour-blindness, and it has been maintained that in the one type the sensation red is absent and in the other type the sensation green. But instances have occurred in which only one eye has been colourblind, the other eye being normal. These instances have belonged to the type which would be classed as redblindness by those who distinguish between redblindness and greenblindness. Now in such cases the colourblind themselves testify that the colours they see with the abnormal eye are yellow and blue, and those they fail to see, red and green. They see the spectrum as composed of yellow and blue, with a grey region in which normal persons see bluegreen.
If we suppose that partial colour-blindness consists in the absence of the sensations both of red and green, we must find some explanation of the difference between the two types which are on the opposite view distinguished as redblindness and greenblindness. In both types it is possible, by mixing in varying proportions light from the shortwaved end of the spectrum with light from the longwaved end, to produce all the colourtones which they are capable of seeing when their retina is affected by intermediate simple lights. In type i. (the socalled redblind), the rays at the extreme end of the spectrum, which give distinct sensations of red to the normal eye, produce no appreciable effect of any kind, and other reddish rays produce only faint sensations. In type ii., the retina is sensitive in some way to rays at the red end of the spectrum; and in general, reddish rays produce more intense sensation of some kind than in type i. In comparing a certain reddish yellow with a yellow almost free from red, the intensity of the reddish yellow light must be made about four times as great for type i. as for type ii., in order that the resulting sensations may be indistinguishable in intensity and colourtone. Clearly there is a great difference in sensitiveness to red light in the two types. But it by no means follows that the red light produces the sensation red in type ii. and not in type i. The most probable explanation is that the red light has a greater power of producing the sensation yellow in type ii. than in type i.*
A corresponding difference is found in normal persons in regard to sensations of yellow. "If by means of a special arrangement we bring a certain amount of the red part of the spectrum and a certain amount of the green part of the spectrum on to the eye at the same time, the result is a sensation of yellow. By the same arrangement we can bring on to the eye at the same time a certain amount of the actual yellow of the spectrum. In this way we can make a match between a mixture of spectral red and green, on the one hand, and spectral yellow on the other, comparing the mixed+ sensation derived from two parts of the spectrum with the sensation derived from a single (yellow) part. We have to adjust the quantities of red light and green light until the mixture seems of the same hue and the same brightness as the yellow, not showing either a reddish or a greenish tone. When this is done it is found that different people differ very materially as to the proportion of red and green, the proportion of the intensities of the two lights, necessary to make the match with yellow."*
* Professor G. E. Miiller has given an elaborate explanation of how this takes place. See Zeitschrift f. Psychologie und Physiologie der Sinnesorgane, Band XIV. Heft 3 und 4, p. 182.
+ The sensation as distinguished from the stimulus is not mixed. Physiologists are apt to confuse the two things.