656 THE EYE IN EVOLUTION 



A specific device developed by certain species in order to increase 

 the visual acuity is the provision of intra-ocular filters. These 

 increase the sharpness of the image in the same way as tinted spectacles 

 do when appropriately chosen : they diminish chromatic aberration 

 largely by eliminating some of the blue and most of the violet rays, 

 while at the same time they cut down the glare and dazzle caused by 

 irregularly scattered light from a bright sky. As would be expected 

 this device is largely confined to diurnal Vertebrates and is not typical 

 of nocturnal types to which the transmission of every available ray 

 is of importance (Walls and Judd, 1933). 



For these optical purposes a yellow filter is the most efficient and 

 is the most widespread optical device found in the vertebrate eye. Thus 

 a yellow cornea is found among Holosteans in Amia, and in a few 

 highly diurnal Teleosteans such as the carp, Cyjirinus, and the pike, 

 Esox ; a yellow lens is found in the lampreys (except the nocturnal 

 Geotria), in the diurnal gecko, Lygodactylus, in some diurnal snakes 

 {Mal'polon, DryojjJiis, etc.), in the tree-shrew, Tujmia, among Insecti- 

 vores, and in most squirrels (Sciuridae, except the nocturnal flying 

 squirrels) ; a yellow pigment is found in the central area of the retina, 

 possibly in the chameleon and certainly in man, converting it into a 

 macula lutea ; and yellow oil-droplets are found in the cones of the 

 frog, Sphenodon, the turtles, diurnal lizards and birds. Finally, as was 

 originally pointed out by Schultze (1867), the blood in the capillaries in 

 the membrana vasculosa retinae of Holosteans, many Teleosteans,^ 

 Anurans and snakes, and in the vascularized retina of the eel and some 

 Mammals ^ must constitute an effective yellow filter through which 

 light must pass to reach the cones. 



We have seen that orange and red and occasionally green droplets 

 in addition to yellow, occur in the cones in the periphery of the retinae 

 of turtles and diurnal birds ; these must aid visual discrimination by 

 - enhancing colour-contrasts.^ 



THE STRUCTURE OF THE RETINA 



Not only does the visual acuity depend on the efficiency of the 

 dioptric system of the eye, but also — and equally^ — on the ability of 

 the retina to act as an analytical receptor. This ability depends 

 essentially on two factors — the fineness of the mosaic of retinal 

 receptors and the degree of summation in this tissue. 



If the simplest pattern of two object-points is to be analysed, 

 each must stimulate a separate receptor element while an intervening 

 element must remain unstimulated. So far as the retinal mosaic is 

 concerned, therefore, the greater the number of visual cells and the 

 closer tiieir packing, particularly in the important receptor area of the 



1 p. 299. 2 p. 479. 3 p g3i^ 



