34 COLOUR-SIGHT AND COLOUR-BLINDNESS 



are equally excited at once, white is the result ; if there is no 

 excitation the sensation is one of blackness or rather absence of 

 white and according to the nerves irritated and the degrees of 

 irritation so are all the colours produced. 



And colour-blindness is supposed to be due to the depravity 

 or absence of one or more of the nerve elements. 



Bering's theory of six fundamental sensations in pairs, 

 though claimed to be trichroinic is surely tetrachromic for ifc 

 supposes four principal and pure colours, red and green, yellow 

 and blue, besides white and black. The pairs are at once 

 complementary and antagonistic and are presumed to act upon 

 three substances which are somewhat similar to, or are constitu- 

 ents of, the visual purple. Rays of light affect metabolic changes 

 — perhaps photo-chemico-vital^on these visual substances in 

 different ways — some promoting constructive changes (changes 

 of assimilation or anabolic) others promoting destructive changes 

 (changes of dissimilation or katabolic) and when these changes 

 are in equilibrium no sensation is experienced. Red is a 

 dissimilative change, green an assimilative ; yellow dissimila- 

 tive, blue assimilative. All the colours act upon the white- 

 black substance, but the red-green does not effect the yellow- 

 blue nor vice versa. 



And colour-blindness is the absence of one or more of these 

 fundamental sensations. 



There are other theories: — Ebbenhaus, based upon the 

 supposed decomposition of the visual purple and Parinaud, upon 

 the action of the rods and cones — stimulation of the former 

 causing a sensation of non-coloured light and of the latter all 

 possible sensations of colour. 



But " no theory is satisfactory in character and no facts up 

 to the present seem sufficient to explain the mechanism of 

 colour vision." As one author says, "this subject has not yet 

 found its Newton." As another says, " no colour-blind retina 

 has been secured for microscopical examination." 



Dr. Hugo Magnus, of Breslau, in 1877, advanced a some- 

 what curious theory : — That primitive man was colour-blind ; 

 starting life with one positive perception, namely, light ; and 

 one negative, namely, not-light, or darkness ; in fact that 

 he saw nothing except light and dark and had no sense of 

 colour. And Magnus recognised four stages in the march of 



