NATURAL THEOLOGY. 



45 



apparatus is so delicate and obscure that they cannot 

 decide upon its true character. 



B. There must be some wonderful mechanism 

 about this aperture, the membrane of the iris must be 

 exceedingly sensitive to feel every change of the light 

 so immediately. 



T. Why, here is another remakable fact, which 

 shews what amazing design runs through every part 

 of our structure. The changes in the pupil are to 

 benefit the retina, so that too much light may never 

 fall upon this tender substance. Now, as the retina 

 is the part to be benefitted, it is evidently desirable 

 that this should have the management of the pupil by 

 which the light is admitted. Had the curtain, or iris, 

 which contains the pupil contracted or enlarged the 

 opening by any sensibility which it has itself to the 

 light ; and this, till recently, has always been suppo- 

 sed to be the fact ; it is easy to see, that the tender- 

 ness of the retina might sometimes be greater or less, 

 than that of the curtain, (for it often varies), — and 

 would consequently be liable to suffer. Do you re- 

 collect any notice of this particular ? 



A. It is said, by a late valuable writer,* to have 

 been ascertained by a very delicate experiment, that 

 if a ray of light be admitted into the eye in such a di- 

 rection as only to strike upon the iris or curtain, with- 

 out passing through the pupil — the pupil is not affect- 

 ed — no change of the aperture takes place ; but if it 

 enter the pupil, and thereby fall upon the retina, a 

 contraction immediately ensues. The sensibility is 



*Dr. J. Ware, of Boston. 



