Reigliard. — Senses and Learning in Fishes 157 



light weak and the air filled with a thick fog which ob- 

 scures objects at a little distance. Imagine shadows to 

 be much reduced in intensity, as they are on a foggy day, 

 and all objects correspondingly flat in appearance. Imag- 

 ine a little way above your head a great mirror extending 

 parallel to the earth. Through a round, overhead hole 

 in this you see in unnatural positions, distorted and rain- 

 bow-fringed, the upper parts of trees and other objects 

 that lie above it. Their lower parts are invisible to you. 

 Imagine that elsewhere, as you look upward, you see only 

 the reflecting lower surface of the mirror and in this, 

 upside down, you see the images of all objects that lie 

 beneath it. You will then have realized something of the 

 difference between the fish's visual world and our own. 



B. stereoscopic Vision and Accommodation in 

 Fishes. But not only is the visual world of the fish 

 different from our own, the eyes through which it is 

 viewed are unlike ours. They are usually without lids 

 so that the fish has always a glassy stare and never 

 closes the eyes in sleep. They are without tear glands 

 or tear ducts for with its eyes always bathed in water 

 the fish has no need to weep. These differences do not 

 affect vision, but there are other differences that do. 

 Our own eyes look forward and cover a relatively limited 

 field of view in which the two eyes see the same objects 

 within the field of distinct vision. Each eye gets a slight- 

 ly different view of any solid object and by the combina- 

 tion of these two views we get what is called stereoscopic 

 vision. Objects appear to us more in relief than they 

 would if viewed by a single eye. In the fish the eyes look 

 outward, so that their fields of view overlap but little. 

 The fish therefore has a field of view much wider than 

 ours. To look about he does not need to turn the body 

 or the eyes to the same extent that we must turn our eyes 

 or heads. But since he views each object (except those 

 included within an angle of 5° in front of his head) , with 

 one eye only — these objects probably lack relief (except 

 within the 5° angle). The fish has then very limited 

 stereoscopic vision, if indeed he has any (Tschermak, 



