146 Experimental Philosophy. [Lecture 10. 



tion disperse the rays of light, and diminish the 

 objects that are seen through them. The va- 

 rieties of these will be described in a subsequent 

 lecture. 



18. Vision is performed by a contrivance of 

 this kind. The crystalline humour, which is 

 seated in the fore part of the human eye, imme- 

 diately behind the pupil, is a perfect convex lens. 

 As therefore every object is rendered visible by 

 beams or pencils of light which proceed or di- 

 verge from every radiant point of the object, 

 the crystalline lens collects all these divergent 

 rays, and causes them to converge on the back 

 part of the eye, where the retina or optic nerve 

 is spread out ; and the points where each pencil 

 of rays is made to converge on the retina, are 

 exactly correspondent to the points of the object 

 from which they proceed. As, however, from 

 the great degree of convergence which this con- 

 trivance will produce, the pencils of light pro- 

 ceeding from the extreme points of the object 

 will be made to cross each other before they 

 reach the retina, the image on the retina is always 

 inverted. 



19. The magnitude of the image painted on 

 the retina will, therefore, it is evident, depend on 

 the greatness or obtuseness of the angle under 

 which the rays proceeding from the extreme 

 points of the object enter the eye. For it is 

 plain, that the more open or obtuse the angle is, 

 the greater is the tendency of these rays to meet 



