212 ADAPTATIONS TO NOCTURNAL ACTIVITY 



the curvature of the cornea and thus have been able to keep the lens 

 from becoming so large and so round as to increase spherical aberration 

 to any disastrous extent. 



The end result of the juggling of these factors is an eye which, as 

 compared with a diurnal eye such as that of man, is : 



1. Relatively large for the size of the animal, and absolutely large if 

 there is room for it in the head — even altered in shape ('tubular' eyes — 

 v.i.) if there is not space enough for an orthodox eye. 



2. Provided with a relatively large anterior segment, making room 

 for a large-opening pupil and a proportionately large lens, which is : 



3. More nearly or even quite spherical and set far back from the 

 cornea (which where convenient is less sharply curved), so that the 

 anterior chamber is often deepened and: 



4. The optical center is far back within the eye, resulting in a smaller 

 and brighter retinal image. 



'Tubular' Eyes — There are certain interesting consequences of these 

 changes which, in themselves, add nothing to the capacity of the eye for 

 operation in dim light. Whereas the diurnal eye tends to have a small 

 anterior segment and a large fundus, the nocturnal eye tends to have a 

 large anterior segment and, the image being small, would gain nothing 

 from having a posterior segment proportioned to it as in a diurnal eye. 

 The result is a relatively small fundus, rendering the eye somewhat 

 tubular in some species in which the anterior segment has become enor- 

 mous. This is true of the owls and their relative Podargus, some lemu- 

 roids, and a majority of the deep-sea fishes which have kept their eyes. 

 These forms, so to say, have ballooned the eye to the point where there 

 is barely room for it in the head (Fig. 84), and have continued to en- 

 large the anterior segment so that the effect is produced of the useless 

 equatorial region of the globe having been cut away (Fig. 136b, p. 400). 

 The eye of the deep-sea fish bears the same relation to a standard-shaped 

 fish eye of the same axial length as does the part of an apple, removed 

 by a cylindrical coring tool, to the intact apple. 



The small size of the retina in tubular nocturnal eyes tends to make 

 more narrow the angle which embraces the visual field outside of the 

 eye. This demands considerable rotability of the eyeball in the orbit, in 

 order that the animal shall be able to see about him through a safely 

 wide angle. But, these tubular eyes have become so large that they are 



