214 ADAPTATIONS TO NOCTURNAL ACTIVITY 



is imaged. Hence the eye with a spherical lens sees its object about as 

 well in the periphery of the retina as in the fundus. A moving object can 

 therefore travel farther alongside or around the head of the animal be- 

 fore the latter need make any movements to keep it in good view. The 

 only extra requirement is a wide cornea, and the net result is a widened 

 visual field. 



Broad Cornece — The eflFect of an extensive cornea — and some, like 

 that of the house-mouse, cover about half the surface of the eyeball — 

 like that of large ocular size as such, is easily misunderstood. As has 

 been made clear, it is not true that a unit retinal area is more brightly 

 illuminated in a large eye {ceteris paribus) than in a small one. This 

 does become true only when the lens and pupil are disproportionately 

 large. Neither does a large cornea let in more light, as is commonly 

 supposed. It is the pupil which regulates the amount of light that reaches 

 the retina. The cornea would not need to be any larger than the fully 

 dilated pupil, if the iris were right against the cornea. To let light 

 rays hit the front part of the retina and increase the periscopy of the 

 eye, however, the cornea must be broader than the pupil; and the more 

 so, the farther the iris and lens are from the cornea. Since nocturnal 

 eyes tend to have deep anterior segments for the reasons given above, we 

 can see that their relatively broad corneas (compare lynx and man in Fig. 

 71, p. 173) are a consequence of these other ocular changes, and do not 

 in themselves promote sensitivity to light. The recession of the optical 

 center into the eye, in strongly nocturnal forms, cannot be wholly com- 

 pensated for by a broad cornea. The deeper the optical center within the 

 eyeball, the smaller and brighter the image will be; but the farther back 

 the center is from the pupil, the larger the pupil and the cornea must 

 become in order to maintain a wide-angled visual field. Despite all efforts 

 of pupil and cornea, the nocturnal eye tends dangerously toward 'tube 

 vision' — that restriction of visual field which we experience in looking 

 through an aperture located before the eye. The nocturnal animal, there- 

 fore, dares not rely solely upon increasing the objective intensity of the 

 image, by manipulating its relative size through mere gross changes in 

 ocular morphology and optics. He must keep the need for such changes 

 minimal (since they inevitably detract from visual acuity and visual 

 angle) by promoting the response to whatever light is available. This 

 necessarily means increasing the sensitivity of the retina itself. 



