192 J. LeConte—Phenomena of Binocular Vision. 
concentric layers, one within another increasing in density and 
in curvature until the central nucleus becomes a very dense 
curious and universal structure. 
Very recently Dr. Hermann (loc. cit.) has discovered another 
optical property conferred upon the lens by this structure—a 
property which he regards as of great importance in perfect 
vision, and which, therefore, he thinks, entirely explains the 
design of the structure. It is the property of forming images 
of olyects lying on the margins of the field of view, far more per- 
feet than could be jormed by a homogeneous lens of the same 
focal distance. And since, as he supposes, perfect images of 
marginal objects necessitates distinct vision of these objects, 
he calls a lens having this structure perzscopic. 
By mathematical discussion he shows that, in a homogeneous 
. lens, while the central rays from radiants near the middle 9 
the field of view—i. e., of pencils nearly parallel with the axis 
of the lens—are brought to a perfect focal point, the central 
rays, from marginal radianis—i. e., of very oblique pencils— 
form crossing caustic lines. The refraction in the former case 
is stigmatic, in the latter astigmatic. Therefore the picture 
formed by such a lens is distinct in the central parts and very 
indistinct on the margins. Now this well known defect of a 
homogeneous lens Dr. Hermann shows is certainly in a great 
measure, probably entirely, removed by the structure peculiar to 
the crystalline lens. For in the case of a lens composed of con- 
centric lamin increasing in density and curvature to the 
center, the astigmatism of oblique pencils is greatly diminished, 
and if the lamine be infinitely thin, probably entirely removed. 
The picture formed by such a lens would therefore be perfect 
in all parts, even to the extreme margins. The crystalline lens, 
therefore, by its structure is endowed with the properly of form- 
ing distinct images of objects lying even on the extreme margins 
of the field of view—of forming perfect images on all parts of 
the retinal screen even to the extreme anterior margins. It is 
this structure, according to Dr. Hermann, which gives the eye 
its enormous field of view, compared with that of optical 
