50 INNERVATION. [CHAP. XV11. 



behind or within the first one by which the light is converged, 

 viz., the concavo-convex one formed by the cornea and aqueous 

 humour, it is adapted to admit the greatest quantity of light to 

 the lens, consistently with the correction of the spherical aberra- 

 tion. 



The aberration of sphericity is further obviated by the increased 

 density of the lens from its surface towards its centre, so that the 

 rays falling on its middle region are made more convergent as they 

 traverse it, than those passing near its border. 



Chromatic Aberration, or that which occasions a coloured image 

 by the inequality of the refraction of the elementary colours of 

 white light by the same medium, is in some way prevented in the 

 human eye, when adjusted to distinct vision. 



The image formed by a convex lens is slightly coloured at its 

 margin. This colouring is corrected in practice by a compound 

 arrangement of lenses differing in shape and density, of which the 

 second, while it continues the convergence of the rays from their 

 original course, re-associates the dispersed colours and recomposes . 

 the white light. 



The achromatism of the eye may be in part due to the diversity 

 of shape and density of the refractive media, which seem to bear 

 some analogy to the system forming the achromatic object-glass 

 of Herschell. This is formed of a double convex lens of crown- 

 glass, with surfaces of unequal curvature, the more convex being 

 turned towards the object ; and of a concavo-convex of flint- 

 glass, the concave side of which receives the lens of crown-glass, 

 while its convex side is towards the eye. The cornea and aqueous 

 humor form a concavo-convex lens which differs in density from 

 the crystalline. 



It is possible that the greater density of the inner fibres of the 

 lens may likewise share in producing the effect. But this entire 

 subject is involved in much obscurity, and it is right to add that 

 some very high authorities, including Sir D. Brewster, deny that 

 the chromatic aberration receives any correction in the eye ; that, 

 in fact, it exists in all cases, and is imperceptible only in con- 

 sequence of its being so slight. It may be observed that when the 

 eye is not adjusted to distinct vision, a coloured fringe is seen 

 around objects. If the eye be fatigued and incapable of adjustment, 

 or if belladonna be used, then colours are seen. 



The rays of light which have now been traced to the retina, al- 

 though they come to a focus in that membrane, yet can scarcely, 

 from its extremely thin and transparent nature, be said to form an 



