Colour-Sensation. 383 



violet in it. Indeed, while Professor Holmgren speaks to the 

 faint shade of violet (' Proceedings of the Royal Society'), Pro- 

 fessor Hippel, who discovered this case and had more oppor- 

 tunities of examining it, states that the blue lines of indium 

 and caesium (which are indigo, not violet) appeared the 

 same to both eyes (Grafs ArcJdv, vol. xxvii.). Therefore 

 some addition is necessary to produce the deeper violet tints 

 of the spectrum; and this can only be obtained by supposing 

 that the violet rays affect the red colour-sense as well as the 

 blue one. 



This hypothesis, that violet results from combining the blue 

 and red colour- sensations, is part of Professor Preyer's theory 

 (see Sect. 38 of his paper in Pfliiger's Archiv), and seems to 

 me to be supported by several other facts. 



(1) As I have already mentioned, when an object is viewed 

 more and more indirectly, so that its image moves from the 

 yellow spot towards the circumference of the retina, sensi- 

 bility to yellow and blue lasts longer than the sensibility to 

 red and green. On the other hand, if the angular magnitude 

 of a coloured object be diminished, sensibility to red and green 

 lasts longer than sensibility to blue and yellow. In each case 

 violet behaves like a compound of blue and red. As the 

 image moves towards the circumference of the retina, the 

 violet object passes through blue into white, the red fading 

 first; while, as the angular magnitude of a violet object is 

 diminished, it becomes reddish (v. Kries, Gesichtsemjifindungen, 

 p. 93). 



(2) Again, Nuel (Annales de V Oculiste, 80, 82, cited by 

 v. Kries, p. 154) describes how in a certain case of acquired 

 colour-blindness "violet appears blue, red and green white." 

 Similarly Schon states that in cases of atrophy of the optic 

 nerve, when green, red, and yellow are no longer recognized, 

 blue alone is correctly designated, and violet is distinguished 

 as dark hlue(Lehre vom Gesichtsfelde, p. 23, cited by von Kries, 

 p. 155). 



(3) I have already mentioned a case of yellow-blue blind- 

 ness described by Stilling, in which blue and violet were, in 

 ft faint spectrum, designated as red, though in a brighter 

 spectrum they seem to have appeared grey. 



All these facts seem to me to point to violet being the 

 result of affecting at once the blue and red colour-senses. 



I am moreover disposed to think that, in addition to the two 

 pairs of complementary colour- senses, there is a fifth colour- 

 sense for white. 



Inasmuch as 



R+G=Y + B = White, 



