STREAMLINING OF THE EYEBALL 379 



those directly overhead. If an object should swing down a semicircle 

 from the zenith toward the horizon, along a meridian of the aerial hem- 

 isphere, it would get shorter and shorter in its meridional length and in 

 its width measured parallel to the surface. Thus even though its linear 

 distance from the fish were constant, its apparent size would become 

 smaller, the closer it approached the horizon. It would be seen more and 

 more dimly, too, for light rays which make small angles with the water 

 are largely reflected, and but little of such light is refracted down 

 through the surface to enter the eye of a fish. 



The entire circumference of the 'horizon', which a swimming man 

 could see by treading water and rotating 360° on his axis, is, for the fish, 

 contracted to the few inches or feet of circumference of his surface win- 

 dow. It follows that a man standing on the bank of a pool is seen as a 

 tiny doll by a fish which is a few yards away and only a few inches below 

 the surface. Our tendency is to suppose that the fish will see us more 

 poorly still, just as we see him less well, if he drops deeper in the water; 

 but since dropping lower enlarges his window, it magnifies objects on the 

 shore — magnifies them, that is, as compared with their apparent size 

 when the window is smaller. To see the fisherman optimally, then, the 

 fish must seek a depth from which the improvement of visibility through 

 enlargement is not cancelled by the loss of light through the greater dis- 

 tance of water through which the rays must travel to his eyes. The poor 

 fish is thus fated never to see us as we are — even through the flat glass 

 side of an aquarium tank. 



Streamlining of the Eyeball — Except in placid, slow-swimming 

 species, the fish eye must ordinarily bear some structural adaptations to 

 its propulsion through the water. The considerable resistance of the 

 medium has two effects upon the eye of a fast-moving fish: friction, tend- 

 ing to scour and erode the corneal epithelium; and asymmetrical pressure. 

 To combat these effects the eye, like the body as a whole, must be stream- 

 lined. The ocular streamlining is of some importance in reducing general 

 bodily water-resistance; for the contribution of the eye, though it may 

 bulge only a bit from the head, is not negligible. The streamlining of 

 the eye affects the eye itself, and helps substantially to maintain the 

 optical status quo: the moving cornea receives added pressure on its 

 advancing nasal border, and at the caudad border of the exposed part of 

 the globe a region of lowered pressure exists as on the upper surface of 

 an airplane wing. These differential pressures would lead to a distortion 



