XII 



SIGHT 



As we have seen, the simplest sense organs respond to touch ; 

 when they are pressed so that work is done upon them and 

 some of the energy from the stimulus passes into them, a 

 physico-chemical change takes place. Similarly the more com- 

 plex sense organs respond only when they receive energy 

 from outside. In the sense of sight the energy in the light does 

 the work ; the energy of the photons or the electro-magnetic 

 waves produces the change, just as it does in the silver salts 

 in the emulsion of a photographic film. 



Thus it comes about that many animals, such as the earth- 

 worms that Maxwell Knight told us about in Chapter II, 

 are sensitive to light and darkness although they have no 

 eyes and cannot see. When light falls upon the light-sensitive 

 sense organs in the skin a change takes place which gives rise 

 to changes in the nerves. The associated nerve impulses are 

 transmitted to the muscles and cause the animals to pop 

 down their burrows into the dark. 



Sight, however, needs much more complicated apparatus 

 to make possible the formation of a mental image that gives 

 a picture of an animal's surroundings. Although there is 

 more than one way of doing this the fundamental principle 

 is dictated by the physical properties of light, which is 

 refracted when passing obliquely between media of different 

 densities, as between air and water — or glass. A piece of 

 glass, for example, with curved surfaces forms a lens which, 

 if its surfaces are convex, throws a picture of what is in 

 front of it on to a screen behind it. The lens of a camera 

 throws a picture on to the sensitive film which is changed 

 chemically by the light according to its intensity so that the 

 picture is recorded. The eye is no more than a little box 

 camera with a lens in front of a sensitive screen that is 

 changed chemically according to the intensity of the light 



