THE SPECIAL SENSES 287 



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

 pecially the crystalline lens, operate upon the 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^ye (Fig. 67). That this actually takes place may 

 be proved by experiment. 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 enable 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 bril- 

 liant, 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 denned, 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 considerable 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 the retina 

 observes no difference, so to speak, between the right and left 

 or the upper and lower positions of objects. In fact, the mind 

 is never conscious of the formation of a retinal image, and 

 until instructed, has no knowledge that it exists. Conse- 

 quently, our knowledge of the relative location of external 

 objects must be obtained from some other source than the ret- 



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 ? 



