44 INNERVATION. [CHAP. XVII. 



Fig - 13 - duct, into the inferior meatus of 



the nose, where it opens under 

 cover of the lower spongy bone. 

 A fold of mucous membrane usu- 

 ally guards its orifice. 



Of the Phenomena of Vision.* 

 The consideration of the changes 

 produced in the rays of light pass- 

 ing through a double convex lens 

 explains, to a great degree, the 

 phenomena resulting from the pas- 

 sage of the rays through the diop- 

 tric media of the eye. 



A ray, falling on the surface of 



Anterior view of the Lacrymal Apparatus. The suc h a } ens j s "b en t towards the 

 lacrymal gland is shown in outline. At the inner 



canthus are the puncta and canaliculi, with the perpendicular to the point of m- 



caruncula between them. The lacrymal sac forms r r 



the upper third of the vertical tube, and the nasal cidenCC, and COntinUCS in that 



duct the remainder. These parts are saparated 



sSmmer V ring ld f tho lining membrane -- From direction to the opposite surface 

 * The following laws affecting the passage of rays of light through trans- 

 parent media of various density ought to be kept steadily in view by the 

 student of the physiology of vision. 



1. A ray of light, in its passage from a rare into a dense medium, is bent, 

 or, in optical language, refracted, towards the perpendicular to the point of 

 incidence, if it fall obliquely upon the surface of the latter medium. The 

 direction of the ray, therefore, is altered in the dense medium. The degree 

 of this refraction depends partly upon the density of the medium, and partly 

 upon the angle at which the ray falls upon it (the angle of incidence). If the 

 ray of light fall upon the transparent surface at right angles to it, it will pass 

 through it without undergoing any change in its direction. 



2. If the ray pass from a dense to a rare medium, it will be refracted from 

 the perpendicular to the point of incidence. 



3. It is obvious, from what has been stated, that the incident and refracted 

 ray will be always on different sides of the perpendicular. 



4. In all cases where rays of light pass from one transparent medium to 

 another, a certain portion of them is reflected at the surface of each new 

 medium. If, therefore, light pass through many different media in succession, 

 much of it will be completely diverted from its original direction by reflexion. 



5. In general, the greater the specific gravity of a body, the greater is its 

 refracting power. 



6. If a pencil of rays diverging from a luminous point fall directly upon the 

 surface of a convex lens, they will not all be equally refracted. The central 

 ray will pass through unchanged in its course. Those nearest the centre will 

 be least refracted ; those most distant from it (which consequently fall with 

 the greatest obliquity upon the surface of the refracting medium) will be 

 most refracted. 



7. On the emergence of the rays of light from a bi-convex lens into air, or 

 any other medium less dense than the lens itself, each ray will be bent away 



