642 THE SENSE OF SIGHT. 



the optic nerve to receive the impression of rays of light, 

 and to communicate them to the brain, in which they excite 

 the sensation of vision. But that light should produce in 

 the retina images of the objects from which it comes, it is 

 necessary that, when emitted or reflected from determinate 

 parts of the external objects, it should stimulate only 

 corresponding parts of the retina. For as light radiates 

 from a luminous body in all directions) when the media 

 offer no impediment to its transmission, 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 apparatus placed 

 in front of it to separate the light of different objects, would 

 see nothing distinctly, but would merely perceive the 

 general impression of daylight, and distinguish it from the 

 night. Accordingly, we find that in man, and all ver- 

 tebrate animals, certain transparent refracting media are 

 placed in front of the retina for the purpose of collecting 

 together into one point, the different divergent rays emitted 

 by each point of the external body, and of giving them such 

 directions 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 humour, the crystalline lens, and the vitreous 

 humour (fig. 178). 



The cornea, the structure of which has been already 

 referred to (p. 637), 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 im- 

 pinge upon the surface in a direction 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 



