APPLICATION OF THE ARGUMENT, 17 



<,ne eye, both these cases were to be provided for ; and for 

 the purpose of providing for them a subtile and appropriate 

 mechanism is introduced. 



I. In order to exclude excess of light, vi?hen it is ex- 

 cessive, 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 great- 

 er or less number of rays at the same time. The cham- 

 ber of the eye is a camera obscura, which, when the light 

 is too sniiiU, can enlarge its opening ; when too strong, 

 can again contract it; and that without any other assist- 

 ance than that of its own exquisite machinery. It is fur- 

 ther also, in the human subject to be observed, that this 

 hole in the eye, which we call the pupil, under all its dif- 

 ferent 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 consideration and con- 

 trivance, to make a circle, which shall continudlly change 

 its diameter, yet preserve its form. This is done in the 

 eye by an application of fibres, i. e. of strings, similar, in 

 their position and action to what an artist would and must 

 employ, if he had the same piece of workmanship to per- 

 form. [Plate II. Fig. 5 6l 6.]* 



II. The second difficulty which has been stated, was the 

 suiting of the same organ to the perception of objects that 



* There is a curious circumstance in the way in which light pro- 

 duces the contraction of the opening of the iris, which strengthens very- 

 much the argument derived from design manifested in its structure 

 and adaptation to its purpose. The object of the iris, it is to be observ- 

 ed, has reference to the quantity of light to be admitted upon the re- 

 tina or expansion of the optic nerve. It is the state of the retina then 

 which regulates the motions of the iris, and it is the action of the hght 

 on the retina which causes those motions and not its action upon the iris 

 itself. This has been shown by a very delicate experiment. If a ray 

 of light be accurately thrown in such a direction, that it shall fall upon 

 the circle of the iris itself, and not pass through its aperture, no con- 

 traction of the aperture takes place ; but if it be so thrown as to pass 

 through the aperture, and fall upon the retina without touching the 

 iris at all, still a contraction of the iris immediately takes place. So 

 that light upon the iris alone occasions no contraction, although it is 

 the part which really contracts when the same light falls upon a dis- 

 tant part. The design here is too obvious to need being enlarged upon. 

 How could the iris acquire the power of contracting when light falls 

 on another membrane, for the protection of that membrane ? although 

 it does not contract when the light falls upon itself alone ? [Ed. 



c 



