SENSE OF SIGHT. 



851 



sensations, and we shall, therefore, have then to consider the mode of 

 production of the visual sensations. 



1. The Dioptric Mechanisms of the Eye. When rays of light 

 proceed from a luminous body they always pass in straight lines, form- 

 ing in their divergence a cone, the apex of which is the luminous body 

 and the base such a plane as may intercept them. So long as the medium 

 is of uniform density rays pass in straight lines, and if they come in 

 contact with an opaque, polished surface they will be reflected, and the 

 angle of reflection is equal to the angle of incidence and lies in the same 

 plane (Fig. 368). If the rays fall perpendicularly to this opaque surface 

 they will be reflected in the same straight line in which they impinged. 

 If the raj 7 s fall upon a translucent surface as they emerge from the 

 opposite side they will be found to be bent from their original course 



FIG. 370. DIAGRAM ILLUSTRATING REFRACTION. (Landois.) 



If Sj D represent a ray of light passing through water, when it emerges at D into the atmosphere it will 

 be bent away from the perpendicular G D and lie in the direction S D. 



through the medium, and though they pass out of the medium in a line 

 parallel with that in which they entered, yet they are not coincident with 

 it so long as the medium is bounded by parallel surfaces (Fig. 369). If 

 these rays pass from a rarer to a denser medium they are bent toward 

 the perpendicular at the point of incidence. If they pass from a denser 

 to a rarer medium tliey are refracted from the perpendicular (Fig. 370). 

 Thus, when an oblique luminous ray passes through a piece of plate- 

 glass its course from the atmosphere is from a rarer to a denser medium, 

 hence it is, in the glass, bent toward the perpendicular; but in passing 

 out it passes from a denser to a rarer medium, hence it is refracted from 

 f the perpendicular, and as the two surfaces are parallel the amount of 

 refraction toward the perpendicular is equal to the amount of refrac- 

 tion from the perpendicular ; therefore the course of the emergent ray is 

 parallel to the course of the entering ray, although not coincident with it. 



