SIGHT 143 



of the eye, the retina, which contains the nerve cells that 

 react to light. These are of two kinds, narrow cylindrical 

 rods and shorter dumpy cones. Many thousands of them are 

 closely packed into a layer at the back of the retina, some- 

 times mixed, and sometimes in separate patches of one sort. 

 When light falls upon them a physico-chemical change is pro- 

 duced which starts an impulse running along the nerve 

 fibres joining the cells to the brain. The change can actually 

 be seen in the rods because they produce a coloured sub- 

 stance, visual purple or rhodopsin, which is bleached to 

 become colourless when light falls on it. 



Behind the rods and cones there is, in many animals, a 

 layer of velvety black colour which absorbs any light that 

 passes on through the retina, and prevents it being reflected 

 back to give a second confusing ghost image, or a scattering 

 of light. It is exactly parallel to the backing put on photo- 

 graphic plates to prevent halation. In many nocturnal 

 animals the black backing is replaced by a reflecting mirror, 

 the tapetum, which reflects any light that goes through the 

 retina. Evidently when there is little light it must all be 

 used for stimulating the rods, and the risk of halation in 

 day vision must be accepted — as someone has said, the 

 tapetum "gives the retina a second chance". Reflection from 

 the tapetum makes the eyes of cats and other animals shine 

 in the dark when a bright light is directed on them. A rather 

 similar but much less bright reflection can sometimes be 

 seen in the eyes of animals that have a black backing and 

 no tapetum, sometimes even in man. This kind of shine, 

 which is pink and not greenish, has a different origin and is 

 caused when a bright ray falls into the eye at just the right 

 angle to be reflected to the observer from the layer of 

 minute blood vessels on the outer surface of the retina — 

 it is light that has not penetrated the retina at all, and its 

 pink colour is the colour of the layer of blood that reflects 

 it. 



Besides the rods and cones the retina contains several 

 layers of nerve cells and a network of small blood vessels 

 that bring nourishment to it. One would expect the rods 

 and cones to be on the surface of the retina so that the 

 image could fall directly upon them, but they are not ; they 



