394 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



saw fewer colours than other people : hence arose the confusion of 

 different colours. . . ." 



Since the time of Goethe a great deal of attention has been 

 devoted to the subject and its literature has attained to formidable 

 proportions. I only propose to consider that aspect of the subject 

 which seems to be of theoretical interest. 



The most obvious distinction between the vision of partial 

 colour-blinds and our own is their inability to perceive differences 

 which are plain to us. As Goethe says, they see fewer colours 

 than we do. This difficulty is of special interest because it is 

 something definite ; it is easy to find out whether a person can 

 distinguish between the effect produced on him by two stimuli 

 which certainly affect us differently, while what the actual nature 

 of the effect is, remains unknown. How, e.g., a certain green light 

 affects a colour-blind eye, by what sensation it is followed, we can 

 only guess ; but we know that the effect cannot be distinguished 

 by the subject from that produced by a certain physical intensity 

 of red light. 



We have thus to deal with a condition in which the conscious 

 responses to varied physical stimuli are fewer than in normal 

 persons ; the question is whether experimental results can be 

 summarised in the simple manner that was possible in the case of 

 normal vision. It will be found that the results appear capable of 

 even simpler description. 



We concluded that, for most experimental purposes, normal 

 colour stimulation is expressible in terms of three colours ; the 

 vision of partial colour-blinds is expressible in terms of two only. 

 If we choose as our fixed lights a red and a blue, these, mixed in 

 suitable proportions, match any part of the spectrum, as the latter 

 appears to the partially colour-blind, and a mixture of the two 

 also matches unanalysed daylight. In the sense in which normal 

 colour vision is trichromatic, this form is dichromatic. 



But, just as in normal persons the actual proportions of the 

 constituents in mixtures equal to various spectral lights do not 

 agree completely, 1 we also find different types of dichromatic vision, 

 distinguished by their respective mixing ratios. The two classes 



1 The question of so-called abnormal trichromatic systems cannot be discussed 

 here. Consult Koenig, Zeits.f. phys. und psych, d. Sinnes., vol. 4, p. 317. Bonders, 

 Arch. f. Anal. u. Phys. (Physiol. Abth.), 1884, p. 518. V. Kries, Zeits. f. Psy. u. 

 Physiol. d. Sinnesorg., vol. 19, p. 63. 



