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CHAPTER VI. 



VISION, 



§ 1. Object of the Sense of Vision. 



To those who study nature with a view to the discovery 

 of final causes, no subject can be more interesting or instruc- 

 tive than the physiology of Vision, the most refined and 

 most admirable of all our senses. However well we may 

 be acquainted with the construction of any particular part of 

 the animal frame, it is evident that we can never form a 

 correct estimate of the excellence of its mechanism, unless 

 we have also a knowledge of the purposes to be answered by 

 it, and of the means by which those purposes can be accom- 

 plished. Innumerable are the works of creation, the art and 

 contrivance of which we are incompetent to understand, 

 because we perceive only the ultimate effects, and remain ig- 

 norant of the operations by which those effects are produced. 

 In attempting to investigate these obscure functions of the 

 animal or vegetable economy, we might fancy ourselves en- 

 gaged in the perusal of a volume, written in some unknown 

 language, where we have penetrated the meaning of a few 

 words and sentences, sufficient to show us that the whole is 

 pregnant with the deepest thought, and conveys a tale of sur- 

 passing interest and wonder, but where we are left to gather 

 the sense of connecting passages by the guidance of remote 

 analogies or vague conjecture. Wherever we fortunately 

 succeed in deciphering any continued portion of the dis- 

 course, we find it characterized by a perfection of style, and 

 grandeur of conception, whicli at once reveal a master's hand., 



