

CHAP. XI.] PERIPHERAL NERVOUS END-ORGANS. 465 



Optical When solutions of rliodopsin are exposed to light, 



characters the colour changes from a purple tint, through red and 



of Rhodop- orange, to yellow Jbefore becoming colourless. According 

 to the rapidity of our observation, therefore, will be our 

 notion of the pristine tint, when we bring the solution into the light 

 to examine it. If our eye fixes it in the red stage first, and then we 

 begin to note the fading, we shall be led to conclude that the original 

 tint was a deep red rather than a violet ; and in fact, many observers, 

 as Boll who proposed for the colouring matter the name 'Seh-Roth', 

 have fallen into this error of description. To obviate this self-decep- 

 tion we must prepare, in the dark, solutions of the visual purple, of 

 strengths becoming progressively weaker, and bring them (one by 

 one in the order of their concentration) into the light. It will be ob- 

 served under these conditions that the tints of the different strengths 

 run from purple-violet (in the strongest solution) through purple-red, 

 carmine-red and rose-colour, to lilac in the weakest. In other words, 

 the fading of the colour on exposure to light is different from the 

 fading of the colour on progressive dilution. In the former case 

 appears a yellow admixture which is absent from the original colour. 

 Indeed it is to some extent a misnomer to speak mostly of a 'fading' of 

 the visual purple, for besides itself fading, the visual purple is converted 

 into a visual yellow, which in its turn fades. The hypothesis that 

 visual purple becomes visual yellow in the sunlight, while visual 

 yellow fades in the same circumstances, suffices to explain all the 

 diversities of tint presented by the retina. According to the rapidity 

 of the conversion of purple to yellow, and according to the rate at 

 which the yellow is dissipated altogether, will be the particular tint 

 of an exposed retina. It will be shewn that different regions of the 

 spectrum have different powers of converting and bleaching rhodopsin. 



spectrum When light is passed through visual purple and after- 



of Visual wards through a prism, there is obtained a spectrum 



Purple and offering no defined absorption bands, but presenting a 

 Visual Yellow. g enera j absorption of rays in the centre of the spectrum, 

 from a little to the red side of D to the violet side of G. Visual yellow 

 blocks the rays from the red side of F to the blue end of the spectrum. 

 The most complete absorption by the visual purple is in the region of 

 E : that by the yellow is at G. 



The characteristic transformation of visual purple in 



tne presence of sunlight opens up a number of in- 

 different teresting questions. Is it to the highly refrangible 

 wave-lengths invisible rays or to the coloured rays that the change is 

 Purple Vl dae ? Are ra y s of a11 degrees of refrangibility in the 



visible spectrum equally concerned in the action? 

 What is the nature of the conversion of purple into yellow; is it 

 physical or chemical, a synthesis or a splitting asunder of complex 

 into simpler groups ? Several of these questions have already re- 

 ceived a satisfactory solution, as we shall now attempt to shew. The 



G. 30 



