360 VISION. LECT. XX. 



LECTURE XX. 



ARGUMENT. Two parts or stages in the process of sensation, viz. the 

 action of external agents, and the perception of impressions. 



Vision is effected by means of the physical agent called light. Necessity 

 for an optical apparatus to form images of exterior objects. Three 

 modes of forming images : the camera obscura ; the mosaic dioptric in- 

 strument; and the concentrating dioptric instrument. 



Structure of the human eye : its membranes and humours ; dimensions of 

 its various parts. 



Mechanism of vision. Action of the eye on the rays of light. Image 

 formed on the retina. Adaptation of the eye to vision at different dis. 

 tances : hypotheses to account for it : Sturm's explanation. Presbyopia 

 and Myopia. Achromatism of the eye. Cause of erect vision with in- 

 verted images. Idea of distance and size of objects. Single vision. 

 Wheatstone's observations on binocular vision : his stereoscope. Dura- 

 tion of impressions on the retina. Ocular spectra: accidental colours. 



Two Stages in Sensation. THE function of every appa- 

 ratus of sensation is composed of two distinct parts : 



1st, external objects produce in the sensorial nerves a 

 modification which is peculiar and specific for each sense ; 



2dly, this modification is transmitted to the brain, where 

 the impression is received and transformed into a perception 

 of the external object. 



Causes of Vision. In vision, as in hearing, our relations 

 to external objects are the same. When the sun rises above 

 the horizon and we see it, there necessarily exists some sen- 

 sible relation between that luminary and our eyes. It must 

 be by the physical agent called light, that the sun produces 



