NATURAL THEOLOGY. 25 



traction of the iris, and consequent enlargement of the pupil, a 

 larger pencil of rays is admitted. It is remarkable that the image 

 formed on the retina must always be inverted, and yet such is the 

 power of habit and experience, derived from touching objects, 

 that we see things as they are in reality, and not as they are paint- 

 ed in our eyes — experience thus correcting the errors of sense. It 

 is in the same way that we see single, though we have an image 

 made in each eye. But if we change the ordinary position of our 

 eye, the habit is broken, and we see double.] 



I. In order to exclude excess of light, when it 

 is excessive, and to render objects visible under 

 obscurer degrees of it, when no more can be had, 

 the hole or aperture in the eye, through which the 

 light enters, is so formed as to contract 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, 

 which w^e call the pupil, under all its different di- 

 mensions, retains its exact circular shape. This 

 is a structure extremely artificial. Let an artist 



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