THE PERCEPTION OF SPACE 



671 



The curvature of the primary oj^tical surface is also of importance 

 since it determines the extent of the sohd angle within which light can 

 be refracted into the eye. Thus the human cornea subtends only 60° 

 of a circle and the visual field averages 150° ; the cornea of the cat 

 forms a much more prominent curve subtending 170° of arc and its 

 visual field averages 200° ; the cornea of the chameleon is largely 

 covered by the lids which leave only a small central aperture roughly 



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WATER 

 SURFACE-- 



FiG. 804. — The Upper Visual Field of a Submerged Fish. 



In the lower figure the fish is seen swimming in fresh water and the paths 

 of the rays of hght are dehneated. 



In the upper figure is shown the view seen by the fish with the central 

 circular window of aerial vision near the periphery of which bodies become 

 progressively foreshortened ; around it is reflected a view of the bottom mir- 

 rored on the surface of the water (after Walls). 



concentric with the pupil so that it is restricted to tubular vision, a 

 disability neutralized by the extraordinary mobility of its eyes (Figs. 

 799 and 845). In under-water vision the cornea is ineffective as a 

 refractive element and the lens serves as the determinant of the visual 

 angle ; for this reason the lens is circular and situated far forward, 

 closely approaching the cornea in fishes, often protruding beyond the 

 level of the surface of the head ; in such an eye the field is determined 

 solely by the angular extent of the retina. The pupillary aperture is 

 a less important factor, but the transversely elongated pupiLs of 



