154 THE SENSES OF ANIMALS 



mostly of water, are virtually incompressible. The eye-coat 

 is so massive that whalers often make a "souvenir" of a 

 whale's eye by scooping out the lens and retina and cutting 

 the back off so that it has a flat surface. They then dry it 

 and use it for an ash-tray — it looks like half a dried turnip 

 with a cup-shaped hollow at the top. 



We have now looked at some of the ways in which the 

 eye works in the vertebrate animals and have tried to under- 

 stand what those animals see. It is fairly certain that although 

 they may look at the same things as we do they do not neces- 

 sarily see the same picture. Most of the mammals see patterns 

 rather than colours, whereas the birds, reptiles and fishes, 

 or many of them, certainly see colours as we do. We will 

 finish this chapter with a brief look at the complexities of 

 colour vision. 



Several theories have been made to explain colour vision 

 but none of them exactly fits all the known facts, so that our 

 understanding of the process is by no means complete. It is 

 probable that the cones of the retina contain substances that 

 are chemically changed by light of different colours, that is 

 of different wave-lengths. According to one theory there are 

 three substances sensitive to red, green and violet respec- 

 tively. According to another there are also three substances 

 but the first is broken down by red and built up by green, 

 the second is broken down by yellow but built up by blue 

 and the third is broken down by white light and built up in 

 darkness. Some of the facts of colour vision fit one theory, 

 some the other, but neither theory fits all the facts, and 

 perhaps eventually some combination of the two will be 

 worked out. 



The great difficulty in studying colour vision is that colours 

 differ in appearance according to the circumstances under 

 which they are seen. If you put a piece of grey paper on a 

 red sheet, and place a piece of tissue paper over the whole, 

 the grey appears to be green ; if the sheet is yellow the grey 

 appears blue ; and vice versa. If you look steadily at a bright 

 red patch for a few moments and then look at a white sheet 

 you will see a patch of green, and so on. Furthermore in 

 everyday life where the shapes of things are irregular and 

 intricate their colours depend upon factors very different 



