SENSE OF SIGHT. 219 



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 about ten millimeters behind 

 the retina. This would be the condition 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 accomplished by the crystalline lens, a body denser than the cornea 



FfG. 34. DIAGRAM SHOWING THE COURSE OP PARALLEL RAYS OP LIGHT PROM, 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. (Front Yeo's " Text- 

 book of Physiology.") 



FIG. 35. DIAGRAM SHOWING THE COURSE OF DIVERGING RAYS WHICH ARE BENT 

 TO A POINT FURTHER FROM THE LENS THAN THE PARALLEL RAYS IN PRECEDING 

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



and possessing a higher refractive power. The manner in which a bicon 

 vex lens focuses both parallel and divergent rays is shown in figures 32 

 and 33. 



The function of the crystalline lens, therefore, is to focus the rays of light 

 with the formation of an image on the retina. 



The retinal image corresponds in all respects to the object from which 

 the light proceeds. The existence of this image can be demonstrated by 

 removing from the eye of a recently killed animal a circular portion of the 

 sclerotic and choroid posteriorly, and then placing at the proper distance 

 in front of the cornea a lighted candle; an inverted image of the candle will 

 be seen upon the retina. The size of the retinal image depends upon the 

 visual angle, which in turn depends upon the size of the object and its dis- 

 tance from the eye. At a distance of 15.2596 meters the image of an ob- 

 ject one meter high would be one millimeter, or a thousand times smaller 

 than the object. 



