INTRA-OCULAR COLOR-FILTERS 191 



(D) Intra-Ocular Color-Filters 



Color vision itself is a potent aid to visual acuity in its broad sense, 

 and was certainly evolved for this application rather than for the 

 aesthetic ones which it has come to have in human vision. But color 

 vision is such a large topic, with so many ramifications, that it needs a 

 long section to itself (Chapter 12). In the present section, we shall con- 

 sider a group of devices which occur only in the eyes of diurnal animals 

 (some, not all, of which have color vision) and promote their visual 

 acuity, and which look at first glance as though they must have some- 

 thing to do with creating color vision — though actually they are just 

 as effective whether their owners happen to be able to distinguish hues 

 or not. 



Types and Distribution — The yellow pigmentation of the human 

 area centralis — making it a macula lutea — was discovered by Soemmer- 

 ing in 1818, In 1840, Hannover first described the oil-droplets which are 

 characteristic of so many vertebrate cones (Fig. 22, p. 54). Some or all 

 of these are always yellow, when any pigment is present in them at all. 

 By 1867, Max Schultze had called attention to the fact that the rich 

 network of capillaries in the inner layers of the mammalian retina con- 

 stitutes an effective yellow screen through which the visual cells must 

 look. In 1887 Schiefferdecker found that in certain fishes the cornea is 

 yellow. (Soemmering, long years before, had seen the color in the pike, 

 but thought it to be in the aqueous humor) . Other species have recently 

 been added to Schiefferdecker's list, and in the past few years it has 

 been found that diurnal squirrels, tree-shrews, snakes, geckoes, and lam- 

 preys (except Geotrid) have yellow lenses. It has been known for many 

 years that the adult human lens is yellow, but not until very recently 

 has it transpired that this is actually of advantage to sharp vision in 

 bright light. 



This imposing list of intra-ocular color-filters exhibits at first glance 

 considerable variety; but (see Table IV, pp. 200-1) they are almost all 

 yellow; and where they are not, they are still of long- wave colors — and 

 they are confined to diurnal vertebrates. It thus appears logical that some 

 inclusive interpretation may hold for all of them, and after a large num- 

 ber of false starts such an interpretation has finally been given. But until 

 a few years ago the macular pigment, retinal capillaries, and yellow comeae 

 were neglected or forgotten, and the yellow lenses went undiscovered for 

 a most surprisingly long time, while attention was fastened upon the 



