72. If a bright color be gazed at intently, and the eyes then be turned to a white surface, a spot will appear; but its color will be the complement of that of the object. Fix the eye upon a red wafer upon a white ground, and on removing the wafer a greenish spot of the same shape takes its place. This result happens because a certain portion of the retina has exhausted its power to perceive the red ray, and perceives only its complementary ray, which is green. The color thus substituted by the exhausted retina is called a physiological or accidental color. In some persons the retina is incapable of distinguishing different colors, when they are said to be affected with "color-blindness." Thus, red and green may appear alike, and then a cherry-tree, full of ripe fruit, will seem of the same color in every part. Railroad accidents have occurred because the engineer of the train, who was color-blind, has mistaken the color of a signal. [Sidenote: 73. The location of the crystalline lens? How supported? Its color and texture? Shape? Size?] 73. THE CRYSTALLINE LENS.--Across the front of the eye, just behind the iris, is situated the _Crystalline lens_, enclosed within its own capsule. It is supported in its place partly by a delicate circular ligament, and partly by the pressure of adjacent structures. It is colorless and perfectly transparent, and has a firm but elastic texture. In shape it is doubly convex, and may be rudely compared to a small lemon-drop. The front face of the lens is flatter than the other, and is in contact with the iris near its pupillary margin, as is represented in the diagram on page 214. It is only one-fourth of an inch thick. [Sidenote: 74. Cataract? Aqueous humor? Vitreous humor?] 74. When this little body becomes opaque, and no longer affords free passage to the rays of light, as often happens {210} 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 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. [Illustration: FIG. 50.--THE RETINAL IMAGE.] [Sidenote: 75. What is a lens and its focus? The miniature image, how produced?]