PHYLOGENY, CHEMISTRY OF FILTERS 199 



Phytogeny and Chemistry of the Intra-Ocular Filters — The old- 

 est of all appears to be the yellow lens, which occurs in lampreys (but 

 not in the nocturnal Geotria) . Here, as well as in snakes and squirrels, 

 the pigment involved (lentiflavin) is soluble in weak alkalies. It is pres- 

 ent in full amount in albino squirrels, hence cannot be scattered melanin, 

 but is a chemically distinct substance (consult Table IV, next page). 



In at least one of the two or three diurnal geckoes (Lygodactylus) , and 

 in the strongly diurnal, squirrel-like tree-shrews (Tupaia) the lens is also 

 yellow though nothing is as yet known about the pigment itself. Presum- 

 ably it is lentiflavin which, since it has been evolved repeatedly in such 

 widely-scattered groups, probably has as its precursor some substance 

 which is present in all vertebrate lenses. 



The most intense colorations of the lens are reached in the ground- 

 squirrels and prairie-dogs, where the lens is almost orange. The lenses of 

 all other American sciurids (excepting the pale ones of the gray squirrel 

 and the colorless ones of the flying squirrels) are alike in color and are 

 matched by a 2 mm. thickness of American Optical Company 'Noviol 0' 

 glass. 'Noviol 0' is matched by the lens of Malpolon monspessulanis, 

 regarded as the most sharp-sighted snake in Europe, and will probably be 

 found to be exceeded in coloration by the lenses of Dryophis and its rela- 

 tives. Other diurnal snakes have paler lenses, the coloration being deep- 

 est in swift, bright-light species such as the racers and whipsnakes. Cre- 

 puscular snakes have little lentiflavin, nocturnal species none at all. Lam- 

 prey, Lygodactylus, and Tupaia lenses compare with those of a gray 

 squirrel or a whipsnake. 



The yellow coloration of the human lens is the result of a precocious 

 aging of the lens nucleus which commences actually before birth, and is 

 thus not on the same footing as that of other yellow lenses. It grows 

 steadily in depth throughout life — the lens of a child has been found to 

 absorb 10% of the blue light entering the eye, that of a 78-year-old man 

 85%. In the normal adult human eye, absorption in the dioptric media 

 increases gradually from the long-wave to the short-wave end of the spec- 

 trum, attaining a value of over 90% in the violet. In old age the spec- 

 trum is cut off in the blue-green region and aged artists find that their 

 blue-containing pigment mixtures look wrong to younger persons, unless 

 the painting is done under an illumination which is particularly rich in 

 short-wave light, such as that from a mercury vapor lamp. The pigment 

 is melanin formed by the interaction of protamine and cysteine liberated 

 by protein-breakdown. The development of the coloration is thus due to 



