Colour-Sensation. 377 



different parts of the retina to different colours, and also as to 

 the effect of diminishing the angles subtended at the eye by 

 small coloured objects; and in both cases red and green 

 colours are found to comport themselves alike, and differently 

 from blue and yellow. When an object is viewed more and 

 more indirectly, so that its image moves from the yellow spot 

 towards the circumference of the retina, sensibility to yellow 

 and blue lasts longer than sensibility to red and green; while, 

 on the other hand, if the angular magnitude of the object be 

 diminished, sensibility to red and green lasts longer than sen- 

 sibility to blue and yellow (von Kries, Gesichtsempfindungen, 

 pp. 93, 95). 



(2) In cases where the colour-senses become affected by 

 disease of the eye, the order in which different colours are 

 found to disappear agrees with the theory of four colour-sen- 

 sations. In cases of atrophy of the optic nerve, it seems 

 pretty clearly established that green becomes invisible first, 

 then red, then yellow, while the perception of blue remains 

 the longest (see Leber, Archiv fur Ophthalmologic, vol. xv.; 

 Leber, Handbuch der Augenheilkunde, vol. v. p. 1039; Schon, 

 Lehre vom Gesichtsfelde und, seine Anomalien). 



In cases where the sight is affected by excess in alcohol or 

 tobacco, Kuel found that green and red became invisible simul- 

 taneously, and blue and yellow later (Annales de V Oculiste i 

 80, p. 110, as cited in von Kries, Gesichtsempfindungen, p. 156). 



On the other hand, the received hypothesis of three colour- 

 sensations does not readily explain how it is that one colour- 

 blind eye sees blue, yellow, and white, and another red, green, 

 and white, as Professors Holmgren and Hippel have found to 

 be the case. If there are only three colour-sensations, it is 

 generally agreed that they must be red, green, and violet. If 

 one of these three sensations were wanting, we should natu- 

 rally expect that the defective eye would have the other two 

 sensations of a normal eye. For instance, if the red sensa- 

 tion were missing, we should expect that the eye would 

 see green and violet, and white would appear of a bluish 

 green complementary to the missing red. Similarly if green 

 were missing, we should expect that the defective eye would 

 see red and violet; while if violet were missing, it would 

 see red and green, and white would appear yellowish comple- 

 mentary to violet. 



The only attempted explanation * I have seen of this pro- 



* Professor Holmgren, when communicating the two cases of one- 

 sided colour-blindness to the Royal Society, states that he considers these 

 phenomena quite consistent with the theory of three colour-sensations ; 

 out he does not explain how he reconciles them with it, except by refer- 

 ring to works which I have not been able to get access to. 



