THE PERCEPTION OF FORM 665 



place, a superlative degree of accommodation may be provided. This is seen 

 in a fish such as the mud-skipper, Periophthaltnus, which can become emmetropic 

 in air using a maximal degree of accommodation. Among land animals a similar 

 excellent accommodation may allow the nevitralization of the hj'permetropia 

 which supervenes on immersion. This applies mainly to representatives of 

 the Sauropsida which employ a well-developed ciliary muscle together with a 

 hypertrophied sphincter mviscle of the iris to mould an unusually soft lens — 

 turtles, water-snakes and birds such as the cormorant. One Carnivore, the 

 otter, Lutra, is capable of a similar accomplishment. 



Apart from this exceptionally high degree of accommodation, several 

 adaptive expedients which we have already mentioned, all of them both interest- 

 ing and ingenious, may be summarized : 



(a) The provision of two optical systems by the use of one or other of the 

 two main axes of a pyriform lens as is seen in Anahleps with its two pupils, or 

 in the kingfisher, Alcedo, with its two fovefe.^ 



(6) Contraction of the pupil either to a stenopoeic opening, as is seen in the 

 sea-snakes (Hydrophinse) or a stenopoeic slit, as in the seals (Phocidse).^ 



(c) The incorporation of the nictitating membrane into the optical system 

 when the eye is immersed, as in diving dvicks, loons and auks.' 



Other optical factors. Apart from the refractive error and its 

 susceptibility to adjustment, the sharpness of the retinal image is 

 influenced by other optical factors. One of the most important of 

 these is the size of the eye, a consideration which essentially deter- 

 mines the size of the image, and therefore the degree of its resolution ; 

 since the size of the visual elements is relatively constant, a larger 

 image stimulates more of them, thus allowing a finer analysis. On 

 the whole, therefore, those animals with relatively large eyes, such as 

 Birds, have the higher visual acuity. In the same way, a flattening 

 of the lens and an approach of this tissue towards the cornea in- 

 crease the distance between the nodal point of the dioptric system 

 and the retina and again increase the size of the image (Figs. 747-8) ; 

 this expedient is well seen in the eyes of Birds and Primates. The 

 small anterior segment with the forward position of the lens and the 

 large globular posterior segment so typical of diurnal birds are 

 excellent examples of this adaptation (Fig. 749). Finally, an efficient 

 pupillary stop to eliminate aberrations by the peripheral part of 

 the lens is of value in increasing the resolution of the image so long 

 as excessive contraction does not diminish the visual acuity by cutting 

 down too drastically the entering light. 



It is to be noted that when the lens is spherical, the aberrations developed 

 in the periphery are less important. This is seen particularly in Fishes in which 

 the refraction of the cornea is eliminated, the lens is spherical with a graduated 

 index of refraction, and the retina practically concentric with the lens 

 (Matthiessen, 1886-93). In such an eye the optical system is practically aplanatic 

 and panoramic, and a pupillary stop is not needed — and is seldom provided. 



1 p. 641. 2 p_ 641. 3 p_ 643. 



