I2 2 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



There are, however, some facts concerning the occur- 

 rence of visual purple which throw light on its function.* 

 It is wanting in night-blind animals, such as snakes and 

 most birds, and is abundant in nocturnal animals, such 

 as most ruminants and all cats, and in owls among birds. 

 Its probable function is to give greater sensitiveness 

 to the impression of simple faint, diffused light, but not 

 to form and color, and is therefore found in the rods, 

 but not in the cones. It is easily destroyed by light 

 and re-formed in darkness, and is therefore specially 

 adapted to feeble light. Hence in very -faint light, but 

 not in full light, at night, but not by day, we detect the 

 presence of an object (though not its form and color) by 

 indirect better than by direct vision. Direct vision is by 

 the cones only (because the image is then on the fovea) ; 

 indirect vision is by rods mostly, and these are made 

 specially sensitive by the presence of the visual purple. 

 This explains also the temporary night blindness of one 

 coming out from a brilliantly lighted room into the night. 

 The restoration of the night vision is the result of the 

 re-formation of the visual purple destroyed by the bril- 

 liant light. 



SECTION IV. 

 Perception of Space and of Objects in Space. 



There is a certain fundamental property of the retina, 

 optic nerve, and associated brain apparatus which must 

 now be explained, for it lies at the very basis of visual 

 phenomena. 



The First Law of Vision. The Law of Exter- 

 nal Reference of Retinal Impressions.— An image 

 is formed on the retina, but we do not see the retinal 

 image. We do not see anything in the eye, but some- 



* Parinaud, Rev. Scientifique, vol. iv, 134, 1895. 



