THE SPECIAL SENSES 299 



74. When this little body becomes opaque, and no longer 

 affords free passage to the rays of light, as often happens 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. 64, 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. 64, v). This humor is a transparent, jelly-like mass, 

 enclosed within an exceedingly 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 convex lens has the 

 property of converging the rays of light which pass through it; 

 and the point at which it causes them to meet is termed its 



FIG. 67. THE RETINAL IMAGE 



focus. If a lens of this description, 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 outside of the window. It will be further noticed that 

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

 the place upon which the image is thrown is much brighter 

 than any other part. 



74. Cataract ? Aqueous humor ? Vitreous humor f 



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



