LIGHT-SENSE 43 



function in these animals may be attributed entirely to the 

 chemical reaction produced by light on the visual purple, 

 which, through stimulation of the rods, starts a nervous 

 impulse. These animals have a dread of light, or photo- 

 phobia; when it appears the moles and hedgehogs at once 

 return to their burrows, the rats retreat to the inner recesses 

 of their caves, and the bats into the darkness of the foliage. 



The photo-chemical reaction of the visual purple, besides 

 providing for perception of light, could easily be amplified 

 to provide for adaptation for different degrees of light, by 

 the extent of the chemical reaction induced varying in 

 accordance with the intensity of the illumination. 



Trendelenburg 13 showed that there is a very close corre- 

 spondence between the curve of scotopic luminosity in the 

 human eye and that obtained from the bleaching of the 

 visual purple of a frog's eye. 



The time which it takes for the visual purple to reform 

 in the rabbit's retina, after it has been bleached, coincides 

 very closely with that which it takes the human eye to regain 

 the approximately full power of dark adaptation after good 

 light adaptation. 



A bleached, undamaged, detached retina will remain 

 bleached, unless brought back into contact with the pig- 

 ment epithelium, when it will regain its violet color. From 

 this, and from their anatomical arrangement, we may 

 assume that the pigment epithelial cells are concerned in 

 the manufacture of the visual purple. They are arranged 

 like the cells in other secreting structures on a basement 

 membrane, immediately beneath which is a rich vascular 



/ 



plexus, the capillary layer of the choroid. The fanciful 

 suggestion made by Edridge Green, 14 that the visual purple 

 is secreted by the rods, has nothing to support it, either 

 anatomically or morphologically. The rods, from their 



