VISUAL ANGLES AND FIELDS 289 



Visual Angles — The angle — or rather, cone — of space subtended by 

 the retina is surprisingly uniform throughout the vertebrates. It is rarely 

 much greater or much less than 170°. This angle is influenced by the 

 angular extent of the retina. If the functional retina comes far forward 

 in the eyeball, as in the horse, the eye may see through an angle much 

 greater than 180°. If the tissue is restricted to the fundus of a tubular 

 eye like that of the owl, the visual angle may be as little as 110°, and 

 is still smaller in deep-sea fishes. 



The visual angle is affected also by the cornea, though not in a way 

 which is self-evident. If the projected area of the cornea in the plane of 

 the limbus be divided into the area of the retina, a quotient is obtained 

 which one might suppose to represent the visual angle. This quotient has 

 been found to be 13.5 for man, 11.5 for a falcon, 10.4 in the pigeon, 

 4.0 in an owl, only 2.5 in a bat. The visual angles of these eyes do not 

 bear such numerical relationships to each other. The cornea-retina quo- 

 tient expresses rather the concentration of light upon the retina and 

 affects the sensitivity of the eye, not its visual angle. If, however, we 

 consider the angular size of a cornea — the portion it includes on a sphere 

 of its own curvature — we have a better indication of the angle of space 

 which that cornea will place upon the retina behind it — provided the 

 retina's own angular size is great enough to receive all of it, which is 

 not always true as for instance in the owls. The human cornea subtends 

 only 60 of a circle with its own radius, and is relatively small. That of 

 the cat occupies 107°. A single human eye sees through 150°, a cat eye 

 through 200 . The bending of the light rays as they pass through the 

 cornea accounts for the apparent discrepancy of the visual angle (which 

 is the effective angular extent of the retina) and the angular size of the 

 cornea. Where the angular size of the retina exceeds that called for by 

 the properties of the cornea, obviously the anteriormost part of the retina 

 must be non-functional. This is true, for example, of the human retina in 

 a zone which extends backward for three millimeters from the ora termi- 

 nalis. This zone is blind, and is said to contain no rhodopsin. 



A very special case is that of the chameleon, whose thick circular lid, 

 fused to the cornea, leaves a crater-like opening the size of the immobile 

 pupil, through which the eye has only 'tube vision' with the whole periph- 

 ery of the retina unable to receive light. One might wonder why the 

 chameleons have not pared away this useless peripheral portion of their 

 eyes as the owls have done. Perhaps it is because they have needed to 

 retain the hemispherical shape of the back of the eyeball to enable it to 



