244 



HUMAN PHYSIOLOGY. 



retina, consists of the cornea, aqueous humor, crystalline lens, and 

 vitreous humor. A ray of light in passing through each of these 

 media will undergo refraction at their surfaces and ultimately be 

 brought to a focus at the retina. Inasmuch as the two surfaces of 

 the cornea are parallel and its refractive power practically the same 

 as the aqueous humor, the media may be reduced to three viz. : 

 i. Cornea and aqueous humor. 

 2.. The lens. 

 3. The vitreous humor. 



The refracting surfaces may also be reduced to three viz. : 



1. Anterior surface of the cornea. 



2. Anterior surface of lens. 



3. Posterior surface of lens. 



The refraction effected by the cornea is very great, owing to the 

 passage of the light from the air into a comparatively dense medium, 

 and is sufficient of itself to, bring parallel rays of light to a focus 



FIG. 32. DIAGRAM SHOWING THE COURSE OF PARALLEL RAYS OF LIGHT FROM 

 A, IN THEIR PASSAGE THROUGH A BICONVEX LENS, L, IN WHICH THEY ARE 

 so REFRACTED AS TO BIND TOWARD AND COME IN A Focus AT A POINT, F. 

 (From Yeo's "Text-book of Physiology/') 



p IG> 33> DIAGRAM SHOWING THE COURSE OF DIVERGING RAYS WHICH ARE BENT 

 TO A POINT FURTHER FROM THE LENS THAN THE PARALLEL RAYS IN PRE- 

 CEDING FIGURE. (Yeo's "Text-book of Physiology.") 



about ten millimeters behind the retina. This would be the condi- 

 tion in an eye in which the lens was congenitally absent. Perfect 

 vision requires, however, that the convergence of the light shall be 

 great enough to allow the image to fall upon the retina. This is 



