WHAT DO ANIMALS SEE? I53 



the eye as a horizontal bar dividing the pupil into upper 

 and lower parts with different optical properties ; when the 

 fish lies at the surface the upper part of the eye is out of 

 water and sees in air while the lower part sees in water. It 

 is difficult for us to imagine what picture of its surroundings 

 Anableps obtains ; perhaps the fish is mainly concerned in 

 detecting movement above or below it and looks at, or 

 attends to, objects in one medium at a time. Another puzzling 

 fish is Toxotes, the Archer fish, which catches insects by spit- 

 ting a drop of water at them as they fly overhead and knock- 

 ing them down on to the water. People have wondered how 

 this fish could allow for diffraction in making its shot, but 

 recent observations have shown that it avoids having to 

 solve this problem by coming very close to the surface and 

 shooting only when the insect is almost directly overhead 

 when diffraction is at its least. Furthermore the blob of 

 water that it shoots quickly breaks up into smaller drop- 

 lets so that in effect the fish is not shooting a single bullet but 

 a charge of shot. Even so, Toxotes' accuracy as a marksman 

 is astonishing. The action is stereotyped; Professor Hediger 

 had a specimen with a deformed mouth so that the shot 

 went to one side of the target, but the fish never learned to 

 allow for this and always scored a miss. 



Even in some aquatic mammals, such as seals and whales, 

 the surface of the eye resembles that of a fish in being un- 

 usually flat, and for the same reason. Seals seem to be able 

 to accommodate to some extent when they come out of the 

 water but it is unlikely that they can do so enough to give 

 a completely sharp image and they are probably then rather 

 long sighted. It is improbable that whales can accommodate 

 for vision in air because they are much more completely 

 aquatic ; nevertheless they do sometimes poke their heads 

 out of the water to look at strange objects such as people in 

 a boat, but they probably do this because the water is ruffled 

 so that they cannot see through the surface. A peculiarity 

 of the whale's eye is the enormous thickness of the outer 

 fibrous coat which must serve to protect the eye in some 

 way. It cannot be to prevent the shape of the eye being 

 distorted by the pressure of the water when the whale dives 

 in pursuit of its food, because the whale's tissues, consisting 



