VISION. 419 



in the skulls of all the mammalia, and perhaps also 

 in those of all other vertebrated animals; affording 

 an illustration of the unity of the plans of nature in 

 the construction of the animal fabric. 



§ 4. Physiology of perfect Vision. 



The rays of light, proceeding from a distant object, 

 strike upon the convex surface of the cornea, which 

 being of greater density than the air, refracts them, 

 and makes them converge towards 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 consi- 

 derably 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 sur- 

 face of the aqueous humour, supposing the cornea 

 not to have intervened. 



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 sub- 

 servient to vision. These next arrive at the crys- 

 talline lens, where they undergo two refractions ; 

 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 sub- 

 stance than either the aqueous 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 



