20 NATURAL THEOLOGY. 



his suiting of his means to his end ; I will not say 

 to display the compass or excellence of his skill 

 and art, for in these all comparison is indecorous, 

 but to testify council, choice, consideration, pur- 

 pose ? * 



To some it may appear a difference sufficient 

 to destroy all similitude between the eye and the 



* The reader will find a comparison, more in detail, between 

 the eye and optical instruments, in the Appendix, No. 17. 



In illustration of the instance adduced here, of the adaptation of 

 the fish's eye to tlie medium in which it lives, we may observe 

 that the powers in the human eye, for example, of drawing 

 the pencil of rays to a focus, and producing an accurate 

 image upon the expanded optic nerve (called the retina, from its 

 net- work structure,) in the bottom of the eye, depends principally 

 upon two circumstances, — the form of the cornea and the con- 

 vexity of the lens. That the cornea may produce this effect, it is 

 not only necessary that it should be convex, (as in the left-hand 

 figure on page 19,) but that the rays should enter it from a rarer 

 medium. As this cannot be effected in the water, the lens or crys- 

 talline humour, which is much denser than water, is brought into 

 operation. In the eye of an animal living in the atmosphere, the 

 lens is removed backwards, and resembles the optician's double 

 convex lens ; but in the fish it is a sphere, and brought in contact 

 with the transparent cornea, it not only has the power to concen- 

 trate the rays of light coming through the water, but by its altered 

 position it increases greatly the sphere of vision. (See the right- 

 hand figure, page 19.) To be critically correct, we may add that 

 it is not exactly the cornea which is deficient in the fish, but the 

 aqueous humour behind it. An aqueous fluid being thus both 

 behind and before the cornea, and that membrane being in a very 

 alight degree thicker in the centre than in the margin, this part of 

 the organ wliich is so efficient in the atmosphere is rendered use- 

 less in water. A man diving, for example, sees imperfectly, being 

 in something worse than the condition of an old man who re- 

 quires spectacles. 



