REFRACTION BY THE CORNEA. 505 



when emitted or reflected from determinate parts of the exter- 

 nal objects, it should stimulate only corresponding parts of the 

 retina. For as light radiates from a luminous body in all di- 

 rections, when the media offer no impediment to its transmis- 

 sion, a luminous point will necessarily illuminate all parts of 

 a surface, such as the retina opposed to it, and not merely one 

 single point. A retina, therefore, without any optical appa- 

 ratus placed in front of it to separate the light of different 

 objects, would see nothing distinctly, but would merely per- 

 ceive the general impression of daylight, and distinguish it 

 from the night. Accordingly, we find that in man, and all 

 vertebrate animals, certain transparent refracting media are 

 placed in front of the retina for the purpose of collecting to- 

 gether into one point, the different divergent rays emitted by 

 each point of the external body, and of giving them such di- 

 rections that they shall fall on corresponding points of the 

 retina, and thus produce an exact image of the object from 

 which they proceed. These refracting media are, in the order 

 of succession from without inwards, the cornea, the aqueous 

 humor, the crystalline lens, and the vitreous humor (Fig. 178). 



The cornea, the structure of which has been already referred 

 to (p. 500), is in a twofold manner capable of refracting and 

 causing convergence of the rays of light that fall upon and 

 traverse it. It thus affects them first, by its density ; for it is 

 a law in optics that when rays of light pass from a rarer into 

 a denser medium, if they impinge upon the surface in a direc- 

 tion removed from the perpendicular, they are bent out of their 

 former direction towards that of a line perpendicular to the 

 surface of the denser medium ; and, secondly, by its convexity; 

 for it is another law in optics that rays of light impinging 

 upon a convex transparent surface, are refracted towards the 

 centre, those being most refracted which are farthest from the 

 centre of the convex surface. 



Behind the cornea is a space containing a thin watery fluid, 

 the aqueous humor, holding in solution a small quantity of 

 chloride of sodium and extractive matter. The space con- 

 taining the aqueous humor is divided into an anterior and 

 posterior chamber by a membranous partition, the iris, to be 

 presently again mentioned. The effect produced by the aque- 

 ous humor on the rays of light traversing it, is not yet fully 

 ascertained. Its chief use, probably, is to assist in filling the 

 eyeball, so as to maintain its proper convexity, and at the same 

 time to furnish a medium in which the movements of the iris 

 can take place. 



Behind the aqueous humor and the iris, and imbedded in 



