Colour-blindness and the trichromatic theory of colour vision. Part II.—incomplete red or green blindness

Abstract

In Part I of this subject I treated of complete colour-blindness in its relation to the trichromatic theory of colour vision. In this communication, which is a continuation of that published, I deal with incomplete colour­-blindness and its relation to the same theory. The number of cases of incomplete red or green blindness is larger than those where the colour­-blindness is complete. In cases of incomplete colour-blindness so far as they have come under my examination the sensation curves of the red and green sensations are similar (in a mathematical sense) to those existing in normal vision, that is to say, if in the normal (say) red curve an ordinate of one colour indicates a perception of “ a ” red, and for the incomplete red-blind a perception of “ b ” red, then in any other position in the spectrum the proportion of normal to incomplete red-blindness is as a : b . This fact leads to a far-reaching conclusion. It tells us that the place of maximum luminosity travels in the case of red-blindness from Standard Scale No. (S. S. N.) 50 to S. S. N. 46. A reference to Table I will show why such travelling of maximum luminosity takes place. To take two examples, a table (Table II) and diagram of luminosity curves for eyes which only perceive one-third of the red sensation and one-third of the green sensation is given. In the first the maximum is closely at S. S. N. 48 ( λ 5720), and in the second at S. S. N. 51 ( λ 5922). The maximum at S. S. N. 49 ( λ 5873) is when the red sensation is about two-thirds of the normal, and at S. S. N. (47) ( λ 5658) when it is about one-tenth of the normal, at S. S. N. 46 ( λ 5600) when there is no red sensation. In the green-blind when there is no green sensation the maximum is closely at S. S. N. 52 ( λ 6000). By observing the position of maximum luminosity we can form an approximate diagnosis of the amount of the defect and as to the sensation in which the defect exists.