224 



ADAPTATIONS TO NOCTURNAL ACTIVITY 



Others in its family have vertical slits or broad, horizontal ovals. The 

 pupil is heart-shaped in Bombina, rhomboidal in some hylids, and may 

 take on still other peculiar forms (Fig. 87). 



The crocodiles, all notorious baskers, have the vertical slit. So does 

 Sphenodon, in which it is tilted a bit out of plumb. The turtles are a 

 diurnal group with insensitive retinae and immobile, circular pupils. Most 

 lizards are diurnal and have round pupils; but several families of lizards 

 are night-prowlers and have vertical slit pupils — among them perhaps the 

 most remarkable of all pupils, that of the majority of the geckoes. This 

 pupil customarily has several tiny notches paired off along its opposite 

 margins. When brightly lit, the pupil closes completely, leaving a series 



Fig. 89 — Pinhole compared with a lens, as a means of forming an image. 



Note that the resolution of the pinhole image depends less upon a critical location of the 

 screen; but the lens image is much brighter since the lens admits more light. 



/- lens; p- pinhole 



position of screen. 



of pinholes formed by the apposed notches (Fig. 88) . Each of these pin- 

 holes is so small that it serves as a stenopaic aperture, forming a sharp 

 image all by itself just as though the lens and cornea were not there, and 

 moreover making accommodation quite unnecessary since it forms fairly 

 sharp images, simultaneously, of objects at various distances (see p. 256 

 and Fig. 89) . Insufficient light gets through any one of the pinholes to 

 stimulate the retina adequately. Since however the images formed by all 

 of them are superimposed on the retina, their total illumination is sufficient. 

 At the same time, the image is sharper than it would be if formed by a 

 single aperture, equal in area to the sum of the pinholes; and no sacrifice 

 of the width of the visual field is entailed (see Fig. 90, especially c). 

 Something of the same effect is obtained by Scylliorhinus (Fig. 91) and 



