530 



COMPARATIVE PHYSIOLOGY. 



an object on the retina, so as to form a well-defined image ; (2) 

 the conduction of the sensory impulses thus generated in the 



Pie. 380.— More advanced stage of development of eye (after Cardiat). a, epithelial 

 cells fonninjg lens, now much altered; b, lens capsule; e, cntaneous tissue about 

 to form conjunctiva; d, e, two lay;ers of optic vesicle, now folded back and form- 

 ing retina; /, mucous tissue forming vitreous humors; g, intercellular substance; 

 A, developing optic nerve; i, nerve-fibers entering retina. 



retina by the optic nerve inward to certain centers ; and (3) the 

 elaboration of these data in consciousness. 



We thus have the formation of an image — a physical pro- 

 cess; sensation, perception, and judgment — physiological and 

 psychical processes. 



In the natural order of things we must discuss first those 

 arrangements which are concerned with the focusing of light — 

 i. e., the formation of the image on the retinal screen. 



