VISION. 163 



therefore proceed to describe the organs of sensation as 

 they exist in the human species. 



VISION. 



" To those," says Mr. Roget, "who study nature with a 

 view to the discovery of final causes, no subject can be 

 more interesting or instructive, than the physiology of 

 Vision, the most refined and admirable of all our 

 senses." 



In a great proportion of the complicated works of 

 creation, although we may be able to see the most 

 admirable mechanism, we are unable to trace its opera- 

 tions, step by step, and point out the ultimate end and 

 object. " But in the subject which now claims our atten- 

 tion," continues Mr. Roget, " we have been permitted to 

 trace, for a considerable extent, the continuity of design, 

 and the lengthened series of means employed for car- 

 rying that design into execution ; and the view which is 

 thus unfolded of the magnificent scheme of the creation, 

 is calculated to give us the most sublime ideas of THE 

 WISDOM, THE POWER, AND THE BENEVOLENCE OF GOD." 



The sense of Vision is intended to convey to us a 

 knowledge of the presence, situation, and color of exter- 

 nal, and distant objects, by means of the light which 

 those objects are continually sending off, either sponta- 

 neously, or by reflection from other bodies. It would 

 appear that there is only one part of the nervous sys- 

 tem, so peculiarly organized as to be capable of being 

 affected by luminous rays, and conveying to the mind 

 the sensation of light ; and this part is the retina, so 

 named from the thin and delicate membranous net- work, 

 on which the pulpy extremities of the optic nerve are 

 expanded, establishing an immediate communication 

 between that part and the brain. 



STRUCTURE OF THE HUMAN EYE. 



In treating on vision, it has been usual, first to trace 

 the optical principles so far as the eye is concerned, and 

 then to apply these principles to the organ itself. But 





