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 those which are refracted more externally, and thereby that defect is 

 obviated which occurs in lenses of uniform density. 



Aherrat ion from parallax may be thus explained. When the object viewed 

 is very distant, the rays of light from it may be considered as nearly paral- 

 lel ; but, when the object is very near, the rays from it diverge considerably 

 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 diverg- 

 ing rays are collected 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 particular dis- 

 tance, 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^ oraccommodatingitself 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 sur- 

 rounding 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 abeiration 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 une(^ually 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 conclude 

 that the eye is perfectly achromatic. 

 880 



