470 THE SENSORIAL FUNCTIONS. 



air, refracts them, and makes them converge to- 

 wards a distant focus. This effect, however, is 

 in part counteracted on their emergence from 

 the concave posterior surface of the cornea, 

 when the rays enter into the aqueous humour. 

 On the whole, however, they are refracted, and 

 made to converge to a degree equal to that 

 which they would have undergone if they had 

 at once impinged against the convex surface of 

 the aqueous humour, supposing the cornea not 

 to have been interposed. 



A considerable portion of the light which has 

 thus entered the aqueous humour is arrested in 

 its course by the iris ; so that it is only those 

 rays which are admitted through the pupil that 

 are subservient to vision. These next arrive at 

 the crystalline lens, where they undergo two re- 

 fractions ; the one at the anterior, the other at 

 the posterior surface of that body. Both these 

 surfaces being convex outwardly, and the lens 

 being a denser substance than either the aque- 

 ous or the vitreous humours, the effect of both 

 these refractions is to increase the convergence 

 of the rays, and to bring them to unite in a focus 

 on the retina at the bottom of the eye. The 

 most considerable of these refractions is the 

 first ; because the difference of density between 

 the air and the cornea, or rather the aqueous 

 humour, is greater than that of any of the hu- 

 mours of the eye compared with one another. 



