638 THE EYE IN EVOLUTION 



visual patterns, the acuity of vision depends on two factors — the size 

 and optical perfection of the retinal image and the resolving power of 

 the retina. So far as optical factors are concerned, the larger the eye, 

 the larger and therefore the more analysable the image ; the more 

 transparent the media and the more perfect the refracting system, the 

 higher its resolution ; and if objects at different distances are to be 

 imaged with equal clarity, an efficient accommodative mechanism is 

 a necessity. So far as the retinal factors are concerned, the denser 

 the mosaic of recipient elements and the lower the ratio of these to the 

 optic nerve fibres (which usually means the greater the number of 

 cones), the higher the resolving power of the retina. Before assessing 

 the importance of visual acuity to the vertebrate world we shall take 

 note of these physical factors on which its effectivity is based. 



THE OPTICAL SYSTEM 

 THE REFRACTION OF VERTEBRATES 



A considerable amount of work has been devoted to the estimation 

 of the static refractive system in Vertebrates, the more important 

 results of which are summarized below. 



CYCLOSTOMES — The lamprey is myopic to the extent of — 8 D in water, a 

 refraction sviitable for a parasitic creature. 



FISHES — It is generally agreed that selachians are all strongly hyper- 

 metropic (in water), the refraction varying from + 8 to + 15 D with some 

 astigmatism (Rochon-Duvigneaud, 1918 ; Verrier, 1928-35 ; Franz, 1931). 



In TELEOSTEANS, however, the position is not so clear. Beer (1894) was the 

 first to study this question intensively ; by retinoscopy he found the eyes of 

 several species to be hypermetropic but he discarded these results in favour of a 

 theoretical analysis of the dioptric system of the eye, which led him to conclude 

 that the teleostean eye showed a degree of myopia varying in different species 

 from —3 D to — 12 D in water (—40 to — 90 D in air), a result confirmed or 

 accepted by most subsequent writers (Franz, 1931). On the other hand, Rochon- 

 Duvigneaud (1918) and Verrier (1928), using retinoscopy vmder water, reaffirmed 

 the presence of a hypermetropia of -f 8 or -f 9 D in a number of species of 

 pelagic fishes, and Verrier (1938), placing a screen in the coats of the eye at the 

 posterior pole, found that a sharp image of a luminous cross could not be obtained 

 at a distance less than 40 cm. under water ; she therefore questioned the 

 accepted view that myopia was characteristic of Teleosteans. Her results were 

 accepted by Rochon-Duvigneaud (1943) in preference to those of Beer ; but 

 the optical problem is difficult and the position is obscure. It would seem 

 probable that some Teleosteans are hypermetroiDic while some may be myopic, 

 particularly deep-sea types wherein a myopia may be present up to — 12 or 

 — 15 D. In this connection it is to be remembered that myopia would be a 

 useful refraction for a fish, for vision under water at any considerable distance 

 is impossible in any case. 



AMPHIBIANS — Among Anurans the refraction of the frog has received most 

 study ; the animal is essentially terrestrial in its visual habits. In air, retinoscopy 

 shows a hypermetropic error of the order of + 3 D with + 2 D of astigmatism 



