Spectrum and the Primary Colour Sensations. 77 



in their nature, and therefore in the conclusions to be drawn 

 from them. This region includes orange as well as yellow. 

 No one doubts that orange is a compound sensation ; and 

 since, according to the curves, the purest yellow has precisely 

 the same effect on the eye, it must likewise be a compound 

 sensation. The same reasoning applies to blue, which also 

 must be compound in its nature. 



The existence of three sharply-defined regions of the 

 spectrum exhibiting curves of the single elevation type, 

 corresponding to red, green, and violet, proves conclusively 

 the existence in the eye of three primary sensations corre- 

 sponding to these colours. The concurrent evidence that 

 yellow and blue are compound sensations completes the 

 proof. 



The evidence presented in this paper for the existence of 

 but three fundamental colour-sensations is entirely the result 

 of experiments made upon the normal retina. This may be 

 supplemented and greatly strengthened by comparison with 

 persistency curves obtained by persons who are colour-blind. 

 In a former investigation * I studied by this method twenty- 

 seven cases of abnormal colour-vision ; and in fig. 22, from 

 that paper, there are shown six out of the seven representative 

 classes into which cases of colour-blindness naturally fall. 

 The persistency curves are drawn after the manner of those 

 in fig. 19; the straight horizontal line at is my own normal 

 curve, and the other compared with it is that obtained by the 

 colour-blind person. These curves were determined when 

 the eyes were not fatigued, but were rested in ordinary 

 daylight. 



In fig. 22 Class I. is a curve with one elevation in the red, 

 indicating red-blindness; Class II. shows one elevation in the 

 green, the characteristic of green-blindness ; Class III. shows 

 two elevations, one in the red and the other in the green, 

 indicating a type of blindness involving those two colours ; 

 Class IV. is a modification of the same type as Class III. ; 

 Class V. has two elevations, one in the red and the other in 

 the violet, another type of colour-blindness involving two 

 colours ; Class VI. has two elevations, one in the green and 

 the other in the violet, still another type, being the third 

 possible variety of colour-blindness involving two colours ; 

 Class VII. is that of a totally colour-blind person, and shows 

 the elevation of the whole curve. These persistency curves 

 for eyes that are naturally colour-blind show exactly the same 

 elevations that are obtained for the normal eye when fatioued 



* " Persistence of Vision in Colour-blind Subjects," Pin*, llev vol xv 

 (1902). 



