24 NATURAL THEOLOGY. 



But further; there are other points, not so much 

 perhaps of strict resemblance between the two, 

 as of superiority of the eye over the telescope ; 

 yet of a superiority which, being founded in the 

 laws that regulate both, may furnish topics of fair 

 and just comparison. Two things were wanted 

 to the eye, which were not wanted (at least in the 

 same degree) to the telescope; and these were 

 the adaptation of the organ, first, to different de- 

 grees of light ; and secondly, to the vast diversity 

 of distance at which objects are viewed by the 

 naked eye, viz. from a few inches to as many 

 miles. These difficulties present not themselves 

 to the maker of the telescope. He wants all the 

 light he can get; and he never directs his instru- 

 ment to objects near at hand. In the eye, both 

 these cases were to be provided for; and for the 

 purpose of providing for them, a subtile and ap- 

 propriate mechanism is introduced. 



[The next figure represents a section of the anterior part of the 

 human eye : — A, A, the iris ; B, the object, from which the rays 

 strike ofFin all directions : a pencil of these enters at the pupil ; a 

 portion is intercepted by the iris A, A. The pencil which enters. 

 the eye, passing through the lens, converges to form the image. 

 But the spaces C, C, are deprived of rays by the intervention of 

 the iris A, A. Yet this in no measure affects the size of the image 

 but only diminishes the intensity of its illumination. By the con- 



another of different density. Dispersion is the separation of the 

 beam of light into differently coloured rays. A piece of glass may 

 differ from another in its power of refracting, and also in its pro- 

 perty of dispersing. It is by duly arranging these different proper- 

 ties that the achromatic telescope is formed. 



