THE PERCEPTION OF FORM 639 



with the axis vertical. If, however, a theoretical correction is made for the 

 difference of level between the reflecting surface of the retina and the layer of 

 rods and cones, Hirschberg (1882) concluded that the hypermetropia became 

 converted into a myopia of the order of — 5 to — 8 D ; Beer (1898), on the 

 other hand, assumed a smaller degree of myopia. Under water, of course, with 

 the elimination of the corneal refraction, a high degree of hypennetropia results 

 (+ 16 D, Hirschberg ; + 25 D, Beer). 



The refraction of Urodeles seems to be suited to their usual environment. 

 Among aquatic Urodeles the new^t is api^roximately emmetropic under water 

 and strongly myopic in air, while terrestrial salamanders tend to be approximately 

 emmetropic in air. 



REPTILES. Little is known of the refraction of Reptiles. In freshly 

 decapitated lizards Kahmann (1932) usually found a low degree of hypermetropia, 

 up to + 6 D in Lacerta agilis. The Crocodilians are slightly hypermetropic in 

 air (+ 7 to + 8 D, Abelsdoi-ff, 1898 ; + 1 to + 2 D, Rochon-Duvigneaud, 

 1943, in the alligator) ; they are therefore highly hypemietropic under water. 

 Most tvirtles are emmetropic, the marine species in water, the terrestrial in air. 

 According to Beer (1898) snakes are usually hypermetropic (up to + 9 D). 



BIRDS. The majority of Birds on which retinoscopy has been undertaken 

 has been found to be emmetropic or slightly hypermetropic, the notable 

 exception being the kiwi, Apteryx, which Is myopic (Rochon-Duvigneaud, 1943) ; 

 the aquatic penguins are«also myopic in air. 



MAMMALS. In the overwhelming majority of Mammals in the wild state 

 the refraction is slightly hypermetropic (under ID); a hypermetropia of greater 

 degree (+ 2 to + 5 D) is found in some Marsvipials, Edentates and Rodents in 

 natural surroundings, while in the many small Mammals equipped with small 

 eyes containing a relatively large lens closely ai^proximating the retina, the 

 hypermetropia may increase up to + 7 or + 10 D. Most of such IMammals (as 

 the mouse) are nocturnal in type and obviously depend visually on the apprecia- 

 tion of differences in luminosity and movement rather than on the very imperfect 

 pattern-vision of which their eyes are capable. The similarly-sized squirrel, on 

 the other hand, which is highly visual in its habits, is practically emmetropic in 

 natural surrovmdings. The vmique eyes of the bats (Chii'optera) are likewise 

 strongly hypermetropic (+ 15 D, Rochon-Duvigneaud, 1943). Myopia in 

 wild and natural conditions is rare and sporadic, being confined essentially to 

 some Primates (mandrils and baboons) and amphibious types — Sirenians (the 

 dugong is — 5 D in air but strongly hypermetropic in water) and Pinnipedes 

 (seals, sea-lions). Thus the seal may have — 4 D of myopia combined with 

 — 9 D of astigmatism with the axis vertical (Johnson, 1901). According to 

 Matthiessen (1886-93) the whale, the eyes of which are fully adapted for aquatic 

 vision, is slightly hypermetropic in w^ater ; in air, of course, it is highly myopic 

 while the asymmetry of the corneal curvature (neutralized under water) results 

 in a considerable degree of astigmatism (4 to 4-5 D). Ungulates are generally 

 emmetropic but tend to have some horizontal astigmatism, perhaps an adaptation 

 to extend the horizontal visual field. Thus most horses are emmetropic with 

 small variations towards hypermetropia, myopia or astigmatism (Rochon- 

 Duvigneaud, 1943). Similarly dogs and cats have a smaller range of refraction 

 than luan ; the majority are emmetropic or nearly so and a high refractive 

 error is a rarity (above -\- 2 D), although errors of the order of 4 D are found 

 more commonly in n:iyopia than hypermetropia. Among domestic animals, 

 however, the refractive error tends to vary considerably 'n\ all directions of error ; 

 thus many animals kept in hutches such as rabbits, guinea-pigs and so on, often 

 develop a high degree of myopia. 



