THE SPECIAL SENSES. 



at the place upon which the image is thrown is much 

 brighter than any other part. 



76. Now all the transparent structures of the eye, but 

 especially the crystalline lens, operate upon its posterior 

 part, or retina, as the convex lens acts upon the paper ; that 

 is, they paint upon the retina a bright inverted miniature 

 of the objects that appear in front of the eye (Fig. 50). 

 That this actually takes place may be proved by experi- 

 ment. If the eyeball of a white rabbit, the walls of which 

 are transparent, be examined while a lighted candle is held 

 before the cornea, an image of the candle-flame may be 

 seen upon the retina. 



77. The form and structure of the crystalline lens endow 

 it with a remarkable degree of refractive power, and en- 

 able it to converge all the rays of light that enter it 

 through the pupil, to a focus exactly at the surface of the 

 retina. When this lens is removed from the eye, as is 

 frequently done for the cure of cataract, it is found that 

 the rays of light then have their focus three-eighths of an 

 inch behind the retina; that the image is four times larger 

 than in the healthy eye, that it is less brilliant, and that 

 its outline is very indistinct. From this we learn that one 

 of the uses of the crystalline lens is to make the retinal 

 image bright and sharply-defined, at the same time that it 

 reduces its size. Indeed, the small size of the image is a 

 great advantage, as it enables the limited surface of the 

 retina to receive, at a glance, impressions from a consider- 

 able field of vision. 



78. As the image upon the retina is inverted, how does 

 the mind perceive the object in its true, erect position ? 

 Many explanations have been advanced, but the simplest 

 and most satisfactory appears to be found in the fact that 



76. How are figures painted upon the retina ? How proved ? 



77. What can be said in respect to the form and structure of the crystalline 

 lens? 



78. How is the inverted image upon the retina presented in its true position to 

 the mind? 



