546 OF SENSATION. 



layer of black pigment, which lines the choroid coat. The purpose of 

 this is evidently to absorb the rays of light that form the picture, imme- 

 diately after they have passed through the retina ; in this manner, they 

 are prevented from being reflected from one part of the interior of the 

 globe to another ; which would cause great confusion and indistinctness 

 in the picture. Hence it is that, in those albino individuals (both of 

 the Human race, and among the lower animals), in whose eyes this 

 pigment is deficient, vision is extremely imperfect, except in a very 

 feeble light ; for the Tascularity of the choroid and iris is such as to 

 give to these membranes a bright red hue, which enables them power- 

 fully to reflect the light that reaches the interior of the eye, when they 

 are not prevented from doing so by the interposition of the pigmentary 

 layer. 



957. The Eye is so constructed, as to avoid certain errors and de- 

 fects, to which all ordinary optical instruments are liable. One of 

 these imperfections, termed spherical aberration, results from the fact, 

 that the rays of light, passing through a convex lens whose curvature 

 is circular, are not all brought to their proper foci, those which have 

 passed through the exterior of the lens being made to converge sooner 

 than those which have traversed its central portion. The result of this 

 imperfection is, that the image is deficient in clearness, unless only 

 the central part of the lens be employed. The other source of imper- 

 fection is what is termed chromatic aberration; and it results from the 

 unequal degree in which the differently-coloured rays are refracted, so 

 that they are brought to a focus at different points. The violet rays, 

 being the most refrangible, are soonest brought to a focus ; and the red 

 being the least refrangible, have their focus at the greatest distance 

 from the lens. Hence it is impossible to obtain an image by an ordi- 

 nary lens, in which the colours of the object are accurately repre- 

 sented ; for the foci of its differently-coloured portions will be different ; 

 and its white rays will be decomposed, so that the outlines will be sur- 

 rounded by coloured fringes. The Optician is enabled to correct the 

 effects of these aberrations, by combining lenses of different densities 

 and curvatures ; so arranged as to correct each other's errors, without 

 neutralizing the refractive power. This is precisely the plan adopted 

 in the construction of the Eye ; which, when perfectly formed, and in 

 a healthy state, forms an accurate picture of the object upon the retina, 

 free from either spherical or chromatic aberration. This is effected by 

 the combination of humours of different densities, having curvatures 

 precisely adapted to the required purpose. 



958. There are certain variations, however, in the conformation of 

 the eye which diminish the perfection of its result. Thus the Cornea 

 may be too convex, and the .whole refractive power too great ; so that 

 the image of an object at a moderate distance is formed in front of the 

 retina, instead of upon it. When this is the case, a distinct image can 

 only be formed, by bringing the object nearer to the eye ; the effect of 

 ivhich will be, to throw the picture further back. Such an eye is said 

 to be myopic, or short-sighted ; and its imperfection may be corrected 

 by placing a concave lens in front of the cornea, of a curvature adapted 

 to neutralize what is superfluous in the convexity of the latter. On 



