196 COLOUR VISION 



greater or less accuracy. First, there is the divergence between photopic 

 and achromatic scotopic vision, a divergence so striking as almost 

 necessarily to hypothecate diverse mechanisms. Second, there is the 

 outstanding triplex relationship between colour and colour-less sensa- 

 tions that three colour-components suffice to arouse almost the whole 

 gamut of colour and colour-less sensations. Third, — and here the relative 

 precision of our knowledge is less — there is a certain opposition evinced 

 by certain colour sensations to each other, between black and white, 

 and between the complementary colours. Observations by the analytic 

 method particularly indicate that there are specially differentiated 

 opponent activities between certain particular colour sensations, such 

 as those of red and green, yellow and blue. 



CHAPTER II 



HISTORICAL REVIEW OF MODERN THEORIES 

 OF COLOUR VISION 



In 1866 Max Schultze^, as the result of anatomical researches, human 

 and comparative, came to the conclusion that the rods were the more 

 primitive organ of vision and were concerned with the perception of 

 light, but were incapable of initiating impulses leading to colour percep- 

 tion. The cones he regarded as more highly differentiated and capable 

 of initiating impulses leading to both light and colour perceptions. 

 The only arguments available to him were (1) the parallel diminution 

 of visual acuity and colour sensations in indirect vision and of the 

 cones in the periphery of the retina : (2) the absence of cones in certain 

 animals of nocturnal habits (bat, hedge-hog, mole, eel, etc. ; relative 

 paucity in the owl) and the paucity of rods in certain animals of diurnal 

 habits (lizard, snake) {v. p. 10). 



The discoveries of the visual purple by H. Mliller (1851) and that it 

 was bleached by the action of light by Boll (1876) were followed by the 

 exhaustive researches of Kiihne (1878). They led Klihne to the con- 

 clusion that we could not only perceive the spectrum by means of the 

 rods and the visual purple, without the intervention of the cones, but 

 that it would under these circumstances appear grey and colourless as 

 it does to the totally colour-blind. He hypothecated the existence of 

 other visual substances to account for the colour phenomena of vision 



1 Arch f. milcr. Anat. n. 1866 ; iii. 371, 1867. 



