THE SENSE OF SIGHT 687 



tically parallel, and as the index of refraction of the aqueous humor is the 

 same as that of the cornea, they may be regarded as but one medium. The 

 refracting surfaces may, therefore, be reduced to the anterior surface of the 

 cornea, the anterior surface of the lens, and the posterior surface of the 



Parallel rays of light entering the eye pass from air, with an index of re- 

 fraction of 1.00025, into the cornea, with an index of refraction of 1.3365- 

 In passing from the rarer into the denser medium they undergo refraction 

 in accordance with the laws of optics and are rendered somewhat convergent. 

 The extent of this first refraction 

 and convergence is sufficiently 

 great to bring parallel rays, if con- 

 tinued, to a focus about 10 mm. 



, behind the retina. This would be 

 the condition in aphakia whether 



j the lens is congenially absent or 



i has been removed by surgical pro- ^ 



; cedures. Perfect vision, however, ^ 2 _ RErRACTION OF HOMOCENTRIC 

 requires that the convergence ot RAYS AND TBE FORMATION OF AN IMAGE. 



sESSSSittSM: trass 



sufficient to focalize them on the 



in every complicated system oi ^ re*a g^ .^ ^ ^.^ 



spheric surfaces there may be assu ^ ^ and 



which the system may ^Sm. either by calculation 



. 



properties be k ^J^ pa th of the refracted ray, and the position 



if those of the object in the first 



medium be known. placed as Gauss showed, by a single 



um e . placed as Gauss showe, y a snge 



ZSSSJSSfJSA * +- -*- * 



-f t tV,p rornea is not parallel to the anterior surface, 



Strictly speaking, the postenor <<*** ^ Ln Aat of the aqueous humor, viz : 

 the index of refraction of . the ~^ ".^rt^ due to the higher index is almost exactly 

 * . 



and e n . .rt ue o 



, 377 . But as the increasn * ^\^ e h{ghl curva ture of tte postenor cornea. 



accurate results - 



