OF SIGHT. 



61 



central ray. The rays composing a pencil falling upon the 

 cornea are refracted in different degrees by the transparent 

 media of the eye, in proportion to the difference between the 

 density of these media and that of the air, and in proportion 

 to the curves presented by their several surfaces. The rays 

 are in the first place refracted by the cornea, by the membrane 

 of the aqueous humour, and by the aqueous humour itself; 

 then, and very particularly, by the crystalline lens, and that 

 differently, by different strata of this body in the ratio of their 

 several densities ; finally, by the vitreous humour ; having 

 passed through which they have come to a focus, and reached 

 the retina at one and the same moment. 



[ 131. "When the object from which the rays of light 

 proceed has extent in space, Fig- 38. 



length and breadth, suppose, for 

 example, that it is the arrow a, 

 b, in fig. 38, then must the ob- 

 ject of necessity appear reversed 

 upon the retina c, d; that which is 

 superior in the object becomes in- 

 ferior, that which is to the right 

 appears to the left in the image.* 

 As every object emits rays from 

 every point in all directions, which 

 then proceed in straight lines, the 

 axal rays e, f, g, of the different 

 pencils proceeding from either end, 

 and the middle of the arrow, a, b, 

 must cross at some point within 

 the eye. Numerous observations 

 satisfy us that this point lies very 

 near the centre of the eye (h), 

 somewhat behind the crystalline 

 lens (#). The prime rays, e, f, g, which proceed from 

 the object may be^named, in reference to the eye, rays of 

 direction, because every prime or axal ray of a pencil de- 

 termines the direction of the* other rays, in order that all of 

 them may meet in a focus upon the retina. The point at 



* [It is most easy to obtain conviction of this reversed position of objects 

 upon the retina, by taking the eye of a white rabbit, free from pigment, 

 clearing the globe from fat, muscles, &c., and then presenting it with the 

 cornea in front to the window ; all the objects before it, such as trees 



