LOOKING THROUGH THE SURFACE 377 



face of the body itself if periscopy — 180° vision for either eye, 360° 

 vision for the two together — is to be attained in a fish whose eyes are 

 back to back (Fig. 128). To many a swift form — the tuna, for example — 

 streamlining is more important than periscopy, and the eye is not allowed 

 to protrude. The broad cornea of a fish eye is not at all related to light- 

 gathering power as it would be in a land animal — the relative size of the 

 pupil, by itself, determines the brilliancy of the retinal illumination (see 

 p. 211). 



The importance of periscopy to a fish is not only seen ecologically, in 

 his increased awareness of near-by prey and increased difficulty of 

 approach by enemies, but is also seen anatomically in his lack of a neck. 

 Despite his buoyancy and rotability on his vertical axis, a fish would need 

 a neck almost as badly as a land animal, were it not for his full visual 

 field. Periscopy has not been important to whales, because vision itself is 

 unimportant to them; but the seals have retained it by keeping the neck 

 flexible — which the whales and sea-cows have not done. 



Looking Through the Surface — When the surface is almost literally 

 as still as glass, an underwater animal can look up through it, but with 

 such peculiar consequences that they may account for the bewildered 

 expression of the average fish! A light ray passing through a rarer 

 medium and striking a denser one will enter the latter from any angle of 

 incidence; but for rays passing from denser media into rarer ones, there 

 is a 'critical angle' of incidence at which they are bent just enough to 

 skim the boundary surface. At greater angles, they are totally reflected 

 and cannot escape into the rarer medium at all. With light, there obtains 

 just the inverse of the situation when a gun is fired at a submerged sub- 

 marine — if the boat is too far away and the angle of fire too flat, the 

 shell cannot enter the water, and reflects, ;'. e. ricochets, harmlessly from 

 the surface. 



The consequence of this is that if a fish looks slantingly upward at the 

 surface, he cannot see through it, but instead sees mirrored upon it 

 objects which are on the bottom at a distance (Fig. 129). If he looks more 

 directly upward, he sees into the air. In effect, there is a circular window 

 in the surface through which he can look (Fig. 129a). This window 

 enlarges if he sinks, shrinks if he rises, but always subtends an angle of 

 97.6° (in fresh water) at his eyes. If the bottom is distant, the surface 

 outside the window is silvery with the reflection of the light scattered in 

 the water, and this light of course always washes over and dilutes the 



