THE PERCEPTION OF FORM 641 



a large lens, while in the retina of such an eye the visual elements are 

 relatively large so that the image still falls on the rods and cones even 

 although it suffers a considerable (relative) excursion. It is probable, 

 indeed, that small eyes (as are typical of the more primitive Placentals 

 which are without accommodative adjustment) have a range of vision 

 as great as the large eyes of most of the more highly developed 

 Carnivores. 



In many cases, however, specific expedients are found which 

 provide a varying degree of accommodative elasticity, some of them 

 probably incidental, others obviously adapted for the purpose. These 

 may concern the optical system of the eye or the retina. Among these 

 the more important are : 



(a) A stenojjoeic jjupil is primarily a protective adaptation against 

 excessive light, but at the same time it converts the optical system of 

 the eye into that of a pin-hole camera in which accommodative 

 adjustment is unnecessary — a simple expedient which, however, suffers 

 from the disadvantage that the available light reaching the retina is 

 cut do\m in proportion as the diminution of the aperture becomes 

 effective. For purely mechanical reasons such a stenopoeic aperture 

 is more readily and therefore more frequently attained by the develop- 

 ment of a slit -pupil than a small circular pupil which requires a 

 difficult muscular effort.^ Since it abolishes the necessity for accom- 

 modation this method is most dramatically employed as a means of 

 overcoming the enormous accommodative adjustment required to 

 bridge the refractive difference between aquatic and aerial vision, as in 

 seals or sea-snakes. ^ 



(b) A duplicated ojitical system may be employed, a device adopted 

 by various amphibious Vertebrates to overcome the large step between 

 aerial and aquatic vision. This is attained by an optical asymmetry 

 of the lens which is pyriform in shape so that it is emmetropic in one 

 axis and hypermetropic in the other. Among Fishes the use of the 

 appropriate system is ensured in Anableps by the presence of two 

 pupils.^ It will be remembered that this fish swims in such a way 

 that the water-line cuts the middle of the cornea ; the upper pupil, 

 subserving aerial vision, admits a pencil of light along the shorter axis 

 of the lens to focus on the lower part of the retina, the lower pupil 

 which is submerged is optically associated with the long axis of the 

 lens and the upper part of the retina (Fig. 766). A somewhat analogous 

 arrangement is seen in the kingfisher, Alcedo, which is j^rovided with 

 two fovea", a central for use in aerial vision, and a second situated 

 in the far temporal periphery somewhat evaginated in an out -pocket 

 of the sclera. The lens is egg-shaped with its narrow end pointing to 



1 p. 612. 2 p. 649. 



^ Compare the dorsal and ventral compound eyes of the wliirligig beetle, p. 244. 



S.O.— VOL. I 41 



