THE NOCTURNAL EYE 211 



would only tend to dim the image on the retina. Doubling the diameter 

 of the eye will double the diameter of the retinal image. This will reduce 

 the illumination per unit area of that image to one-fourth. But suppose 

 the pupil enlarges in proportion to the whole eye. Doubling its diameter 

 will increase, by four times, the amount of light it admits. The illum- 

 ination of the retina will thus have the same strength in any and all 

 eyes whose proportions are exactly the same, regardless of their abso- 

 lute sizes. 



An eye which is simply larger will not, then, have brighter images and 

 greater overall sensitivity in dim light. But enlarging the pupil more yet, 

 out of proportion to the size of the eye, will brighten the image. If the 

 pupil is enlarged the lens must be broadened too, if spherical aberration 

 is not to be increased. A broader iris (to make room for a larger pupil) 

 and a broader (and proportionately thicker) lens will, in themselves, call 

 for an increase in the absolute size of the eye if it is to remain mechan- 

 ically and biologically in balance. We have arrived, by a rather devious 

 route, at a justification for advising our nocturnally-inclined animal to 

 enlarge his eyes — and to enlarge them in a disharmonic manner. 



Enlarging the lens 'out of proportion' to the eye moves the optical 

 center of the cornea-lens apparatus backward (Fig. 71, p. 173). When the 

 curvature of the cornea, lens, or both is now sharpened to keep the image 

 from receding behind the retina, we find that the anterior chamber has 

 deepened and the image has shrunk. This shrinkage of the image is fine 

 up to a certain point, for it accomplishes what was wished : that bright- 

 ening of the image which lets the eye operate in dimmer light. The retina 

 of such an animal being poor in cones, visual acuity is low enough in all 

 conscience already, but it may suffer too much unless now the eye is en- 

 larged harmonically still further, to spread the image without detracting 

 from its brightness. That species is fortunate which has head-room for 

 the development of sensitivity through eye size alone. The cat has a 

 large eye for its size, but a proportionately small retinal image — only 

 38% of the diameter of that of the horse, whereas the diameter of the 

 eyeball is 50% of that of the horse. The human ocular axis is only 1.19 

 times that of the cat, but man's retinal image is 1.37 times as broad as the 

 cat's. Some small, small-eyed animals have had to do the whole job by 

 making the lens spherical, the cornea perhaps remaining broadly curved 

 since the lens has more to do with pulling backward, into the eye, the 

 optical center whose distance from the retina determines the size of the 

 image. The large-eyed carnivores such as the cats have greatly sharpened 



