152 THE SENSES OF ANIMALS 



depth there is no colour difference between a boiled lobster 

 and a live one — both appear to be black. 



A fish's-eye view, if the fish is near the surface, must be 

 very peculiar. In deep water it is probably similar to what 

 we should see if we were at the same depth with an aqua- 

 lung. But near the surface, as in a river or pond, a fish can 

 sometimes see above the water as well as below. It can see 

 into the air above it and to its sides as far out as a line from 

 its eye would make an angle of 45 degrees with the surface. 

 There is thus a dome above it within this limit in which it 

 sees objects in the air. But owing to the bending of light- 

 rays when they enter the water from the air, the base of the 

 dome is in effect much wider and the fish may thus be able 

 to see things beyond the edge of the bank. There is a further 

 complication; beyond the 45-degree angle light is totally 

 reflected from the surface so that to the fish it appears as a 

 bright mirror, and in that mirror it can see the reflection of 

 things on the bottom, to its side and below it. The fish's-eye 

 view must therefore be a complicated picture and it is un- 

 likely that the fish can pay attention to more than a part 

 of it at any moment. We know from the structure of the 

 retina that the acuity of the fish's vision is good, and it is 

 probable that the fish ignores most of the picture but is very 

 quick in responding to any movement that occurs in its 

 visual field. All this can happen only when the surface of 

 the water is still — the moment it is ruffled by wind or wave 

 the surface appears only as a hammered glass mirror would 

 to us, a bright and shiny irregular surface through which 

 nothing can be seen and which gives no sharp reflection of 

 the objects below it. 



It is convenient for fish that the bending of light rays in 

 passing from water through the surface of the eye is less than 

 in passing from air into an eye. Consequently the surface of 

 a fish's eye can be much flatter than that of an air-living 

 animal, and thus blend in well with the general streamlined 

 shape of the fish without the necessity of a vulnerable 

 bulge. 



The most extraordinary eye found in fishes, or among all 

 the vertebrates for that matter, is that of a little tropical 

 fish Anableps, the "Four-eyed Fish". The iris extends across 



