The Perception of Colour, 31 



of the most eminent physiologists. It is certain, however, 

 that histology gives no support to the theory of three or 

 more distinct percipient elements existing together in all 

 parts of the retina, all the rods and cones in one part of 

 the retina of the same animal being of similar construction, 

 so far as can be shown by the microscopes at present in use. 

 A difference in the index of refraction of different 

 elements would perhaps be sufficient, without any dif- 

 ference of form ; but that is merely another hypothesis 

 framed to obviate a difficulty in accepting an opinion 

 which is itself hypothetical. A simpler, and therefore more 

 feasible, view of the phenomena of colour perception is to 

 regard it as the result of photo-chemical changes in the 

 retina ; though, in the present state of our knowledge, it 

 may be somewhat premature to attempt to apply it for the 

 explanation of all the peculiarities of that function, normal 

 and abnormal. In my last communication the suggestion 

 could only be ventured that the retina-purple may serve in 

 some way for the perception of colours. The great difficulty 

 then lay in the circumstance that Boll and Klihne agreed in 

 stating, that the colouring matter was not to be found in 

 the cones; and yet the "inacula lutea is the part of the retina 

 most sensitive to colour, that sensitiveness being most 

 marked in the fovea centralis, which contains only cones 

 and no rods. There are sufficient reasons, however, for 

 supposing that there was error in denying the presence of 

 retina-purple in that region, or in the cones generally. The 

 layer of pigment cells on which the rods and cones rest is 

 the source of supply of the purple, which it seems to 

 manufacture and store up. Now these cells are more abun- 

 dant behind the yellow spot than at any other part of the retina. 

 Dr. Schmidt-Rimpler has reported (Archiv fiir Ophthalmolo- 

 gic, xxi., 3, 1876) that in perfectly fresh human eyes he found 

 the macula lutea of a reddish-brown colour, which gradually 

 faded, giving place to the usual yellowish hue ; the last 

 speck of red, however, being seen in the centre of the fovea. 

 That Klihne did not detect the red colour in the cones is 

 probably to be explained by the delicate points of these 

 structures allowing of its more rapid disappearance than 

 from the broader based rods ; this explanation being made 

 more probable by the fact that the transformations of the 

 retina -purple under the influence of light go on slowly, and 

 are therefore most easily observed in the amphibia and car- 

 tilaginous fishes, whose retinal rods are unusually large. It 



