24 NATURAL THEOLOaY. 



or dilate itself for the purpose of admitting a greater or less 

 number of rays at the same time. The chamber of the eye 

 is a camera-obscura, which, when the light is too small, can 

 enlarge its opening ; when too strong, can again contract it ; 

 and that without any other assistance than that of its own 

 exquisite machinery. It is farther also, in the human sub- 

 ject, to be observed, that this hole in the eye wliich we call 

 the pupil, under all its different dimensions, retains its exact 

 circular shape. This is a structure extremely artificial. Let 

 an artist only try to execute the same ; he will find that his 

 threads and strings must be disposed with great considera- 

 tion and contrivance, to make a circle which shall continu- 

 ally change its diameter yet preserve its form. This is done 

 in the eye by an application of fibres, that is, of strings sim- 

 ilar, in their position and action, to what an artist would and 

 must employ, if he had the same piece of workmanship to 

 perform. 



II. The second difficulty which has been stated was the 

 suiting of the same organ to the perception of objects that 

 lie near at hand, within a few inches, we will suppose, oi 

 the eye, and of objects which are placed at a considerable 

 distance from it, that, for example, of as many furlongs — 1 

 speak in both cases of the distance at which disthict vision 

 can be exercised. Now this, according to the principles ol 

 optics, that is, according to the laws by which the transmis- 

 sion of light is regulated — and these laws are fixed — could 

 not be done without the organ itself undergoing an alteration, 

 and receiving an adjustment that might correspond with 

 the exigency of the case, that is to say, with the different 

 inclination to one another under which the rays of light 

 reached it. Kays issuing from points placed at a small dis- 

 tance from the eye, and which consequently must enter the 

 eye in a spreading or diverging order, cannot, by the same 

 optical instrument in the same state, be brought to a point, 

 that is, be made to form an image in the same place, with 

 rays proceeding from objects situated at a much greater dis- 



