438 SPECIAL PHYSIOLOGY. 



power of the lens is, by some, estimated as high as 1.45. Rays of 

 light passing from one medium to the other, within the eye, are not 

 refracted, according to the above-mentioned coefficients, which refer 

 to the refractive powers of the several parts, in regard to rays passing 

 from a vacuum. The cornea first refracts the rays, and the aqueous 

 humor may be taken as a part of one system with it; the lens acts as 

 a second system, and the vitreous humor as a third. The rays, on 

 entering the cornea from the air, are powerfully refracted towards the 

 perpendicular ; in passing from the aqueous humor into the lens, they 

 are again refracted towards the perpendicular, but only in proportion 

 to the relative index of refraction of the several media, found by di- 

 viding the greater by the smaller coefficients ; on escaping from the 

 lens into the vitreous humor, they are refracted slightly from the per- 

 pendicular, according to the relative index of refraction of those two 

 parts. These facts, the varying refractive powers of different por- 

 tions of the lens, and the not absolutely perfect centring of the sev- 

 eral systems of refracting media in the living eye, render it impossible 

 to obtain mathematical exactness as to its dioptric action. 



The manner in which the eye, regarded as an optical instrument, 

 corrects the .errors or aberrations to which such instruments are liable, 

 is most remarkable. 



The density and refractive power of the crystalline lens are gradu- 

 ally diminished towards its borders, so that the tendency to over-re- 

 fraction in that portion of it is counteracted, and thus, as well as by 

 the aid of the peculiar curves of its two surfaces, spherical aberration 

 is probably absolutely corrected. The dispersion, or decomposition 

 of light in the eye, is very slight ; and different and mutually correc- 

 tive dispersive powers of the cornea, the aqueous humor, and the lens 

 with its capsule, probably correct chromatic aberration. It should be 

 added, that the exclusion of the marginal rays by the iris, diminishes 

 the tendency to both the preceding defects. But under certain cir- 

 cumstances, chromatic vision, or the perception of colored fringes at 

 the margins of objects, occurs ; for example, when the eyes are not 

 correctly accommodated to a near object ; also when one-half of the eye 

 is covered by a dark screen. In the latter case, the corrective effect 

 of one-half of the lens on the other half, by its opposite dispersive 

 influence over the rays of light radiating from any given point of the 

 object, no longer takes place. 



The errors of distortion and confusion are likewise diminished by 

 the exclusion of the marginal rays by the iris ; but they are not 

 noticeable in vision through the central part of the eye, nor in lateral 

 vision. The error from curvature is corrected in the eye by the con- 

 cave shape of the retina. 



The optic centre of the eye, is a point in its antero-posterior axis at 

 which the rays of light intersect each other as they cross to produce 

 an image ; and the distance between this point and the retina must 

 be adjusted to accommodate vision for objects at different distances. If 

 one eye be closed, and the other be turned towards two objects 

 placed one in front of the other, in the same line, and at a certain 

 distance apart, the rays proceeding from the two objects, meeting at 



