ORGANS OF SENSE. 67 



ing one or more glass lenses, which project on the sensi- 

 tive plate the luminous rays coming from the object to 

 be photographed. 



The eye, lined with the choroid, is comparable to the 

 camera. The crystalline lens is the objective, for it is a 

 convex lens, and the nervous membrane, the retina, is 

 the sensitive plate on which the images of objects before 

 the eyes are reproduced. 



In normal vision the form of the crystalline lens is 

 such that the luminous rays passing through it make 

 their sharpest image on the retina. Short-sightedness is 

 due to too great a convexity of the different lenses that 

 the light traverses before reaching the retina ; the image 

 is then produced in front of the retina, and objects are 

 not distinctly perceived unless they are placed near the 

 eye. Far-sightedness depends, on the contrary, on too 

 great flatness of the lenses of the eye : the image would 

 be more distinct behind the retina, and objects can be 

 seen most distinctly when at some distance. Under 

 ordinary conditions the distance at which small objects 

 are most distinctly seen without straining the eyes is 

 ten inches. This distance may fall to less than eight 

 inches in near-sightedness, or may be as great as thirty 

 inches in far-sightedness. Myopia or near-sightedness 

 may diminish as age increases, but presbyopia or far- 

 sightedness always increases. These two infirmities are 

 corrected by the use of glasses, either concave or convex, 

 as the fault of the crystalline lens is too great or too 

 little convexity. 



