VISION. 159 



therefore proceed to describe the organs of sensation as 

 they exist in the human species. 







VISION. 



479. " To those," says Dr. 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." 



430. In a great proportion of the complicated works of 

 creation, although we may be able to see the most admi- 

 rable mechanism, we are unable to trace its operations, 

 step by step, and point out the ultimate end and object. 

 " But in the subject which now claims our attention," 

 continues Dr. Roget, " we have been permitted to trace, 

 for a considerable extent, the continuity of design, and 

 the lengthened series of means employed for carrying 

 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 idea of THE WISDOM, 



THE POWER, AND THE BENEVOLENCE OF GOD. 



481. 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 system, 

 so peculiarly organized as to be capable of being affect- 

 ed by luminous rays, and conveying to the mind the sen- 

 sation 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, es- 

 tablishing an immediate communication between that part 

 and the brain. 



STRUCTURE OF THE HUMAN EYE. 



482. In treating of 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 it 

 is difficult to make these principles understood without 



