210 THE SPECIAL SENSES. 



with the advance of age, an affection termed " cataract" is 

 produced. Between the crystalline lens and the cornea is 

 a small space which contains the aqueous humor (see Fig. 

 48, A). This humor consists of five or six drops of a clear, 

 colorless liquid very much like water, as its name implies. 

 That part of the globe of the eye lying behind the lens is 

 occupied by the vitreous humor, so called from its fancied 

 resemblance to melted glass (Fig. 48, v). This humor is a 

 transparent, jelly-like mass, enclosed within an exceed- 

 ingly thin membrane. It lies very closely applied to the 

 retina, or nervous membrane of the eye, and constitutes 

 fully two-thirds of the bulk of the eyeball. 



75. The Uses of the Crystalline Lens. A con- 

 vex lens has the property of converging the rays of light 

 which pass through it ; and the point at which it causes 



FIG. 50. THE RETINAL IMAGE. 



them to meet is termed its focus. If a lens of this descrip- 

 tion, such as a magnifying or burning-glass, be held in 

 front of an open window, in such a position as to allow 

 its focus to fall upon a piece of paper, it will be found to 

 depict upon the paper a miniature image of the scene out- 

 side of the window. It will be further noticed that the 

 image is inverted, or upside down, and that the paper 



75. What is a lens and its focus ? The miniature image, how produced T 



