546 



PHYSIOLOGY 



have been made out. In the first place it is found in association with the 

 rod retinal structures only, and is therefore absent from the human fovea 

 centralis. It is bleached on exposure to light, both in the retina and also in 



I. 



II. 



in 



Kleetrieal variation in frog's eye as recorded by the >trin<: galvanometer. 



KlN nin\ l.\ and .lou.Y.) 



I. "ii exposure to a single Hash ; II, on exposure to light of nioderat, duration : 

 III. ellert on a light e \ e of momentary darkening. 



solution. The absorption of light by visual purple is ureatest for the middle 

 <>! the spectrum ; the transmitted colour is therefore composed principally 

 of red and violet rays. Small variations of colour are found however in 

 samples obtained from different animals; some are even rose pink in 

 colour. In the second place the most potent colours in producing 

 bleaching are those near the centre of the spectrum, that is the rays 

 which are most strongly absorbed. This pigment therefore obeys 

 Draper's law, which states that those rays that are absorbed are those 

 which produce chemical change. 



