136 ANIMAL MECHANISM. 



rays on the sensible retina. This could not take place in 

 many aquatic anmials, in whose eyes the sclerotica is 

 perfect bone. Another opinion is, that the eye is at rest 

 in looking at near objects, and laboring, when viewing 

 things at a distance. One writer is of the opinion that 

 the iris contracts, and so draws the circular margin of 

 the cornea, towards the pupil, as to make it more* or less 

 convex, according to circumstances. A great variety of 

 experiments have been instituted, to determine, accu- 

 rately, whether there really is any change made in the 

 length of the axis of the eyeball or not, but none of them 

 can be certainly relied upon. A favorite .theory has had 

 its advocates, that the crystalline lens has an inherent 

 power of altering its degree of convexity, and thus ac- 

 commodates the eye to all distances. Of all the absurd 

 hypotheses on the subject under consideration, this is de- 

 cidedly tha most objectionable, in the estimation of an 

 anatomist. The truth is, an action takes place in the 

 eye, in adapting itself to near and distant objects, which 

 depends on that vital property of a living system which 

 no theory can reach, and which the deductions of human 

 philosophy can never with certainty explain. 



Having now completed this very brief and imperfect 

 'sketch, of the mechanism and philosophy of the eye, 

 condensed from manuscript observations, which from the 

 peculiar nature of my studies, have been continually 

 accumulating, I leave it with regret, conscious of its 

 defects, although a hope is entertained that it will 

 serve to excite an additional interest in this department 

 of science, and thus accomplish one of the principal 

 designs of the series to which it belongs. 



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