174 ANTHROPOLOGY. 
spherical, but elliptical, and it is more convex posteriorly than in front. Its 
density also increases from the surface to the centre, and, as the refractive 
power is proportioned to the density of the medium, so the rays which pass 
through and near the centre are brought to a focus sooner, and thus accord 
with iet which are refracted more externally, and thereby that defect 1 is - 
obviated which occurs in lenses of uniform density. 
Aberration from parallax may be thus explained. When. the object 
viewed is very distant, the rays of light from it may be considered as nearly 
parallel; but, when the object is very near, the rays from it diverge con- 
siderably in their course to the eye. The effect of refraction on the distant 
or ‘parallel rays is to bring them to a focus very near the lens; but the near 
or diverging rays are pallcmel into their focus at a greater distance from 
it. The more remote the object, the nearer will the focus be to the lens, 
and for every distance of an object there is a corresponding focal distance 
behind the lens. If, therefore, the eye be adapted for vision at one par- 
ticular distance, the images on the retina of objects at any other distance 
ought to be confused, because the foci will be formed either before or 
behind the retina. In the latter case this membrane will interrupt the rays 
in their course, and in the former it will not receive them until they have 
crossed each other in passing through their focus. This optical defect is 
counteracted by a power which the eye possesses, named adjustment, or 
accommodating itself to vision at different distances. The immediate agency 
in this power is not exactly ascertained, but most probably it depends on a 
vital energy of some of the textures in the globe. It has been ascribed by 
some to the fibres of the lens being muscular, and capable of altering its 
form, density, and distance from the retina; by others to a change in the 
convexity of the cornea, or to an alteration in the form of the globe by the 
compression of the surrounding muscles, or to a change in the position of 
the lens through the action of the iris and ciliary body, or through the 
contraction or erection of the ciliary processes. 
Chromatic aberration depends upon the fact that rays of white light are 
composed of differently colored rays, red, orange, blue, &c., which are 
partly separated or dispersed by refracting media, and, as some colored 
rays are more refrangible than others, they will converge sooner ; thus blue 
and violet are more refrangible than red or orange, and will sooner be 
brought to a focus, and thus the distinctness of the image will be impaired 
or confused, and fringed with different tints. This defect, which is termed 
chromatic aberration, is obviated in the eye by the employment of several 
refracting media, each of different density, and even of different chemical 
composition. Thus the lens has two unequally convex surfaces, each of 
which differs in density from its more central portions; the cornea and 
aqueous fluid form a refracting medium of different consistence from the 
lens or vitreous humor, and it is probable that the dispersive power of these 
may be disproportionate to their refracting effect, whereby an achromatic 
combination is established in the eye, as is effected in optical instruments, 
by combining lenses of different materials; we are not, however, to con- 
clude that the eye is perfectly achromatic. 
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