THE SENSE OF SIGHT. 



161 



viz : i. 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 about 10 millimetres 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 that the image may fall upon the retina. 

 This is accomplished by the crystalline lens, a body denser than the cornea 

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

 lens focuses both parallel and divergent rays is shown in the following 

 figures : 



FIG. 22. 



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 BEND 



TOWARD AND COME TO A FOCUS AT A POINT F '.From Yeo' s Text-Book of Physiology. 

 FIG. 23. 



DIAGRAM SHOWING THE COURSE OF DIVERGING RAYS WHICH ARE BENT TO A POINT 

 FURTHER FROM THE LENS THAN THE PARALLEL RAYS IN PRECEDING FIGURE. From 



Yea's Text-Book of Physiology. 



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 



