330 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



The blindness in the darkened room occurs when the visual yellow is being 

 resynthesizcd to visual purple. 



Theory of Color Vision. Rod cells are better understood than cone cells, 

 but the latter arc known to contain visual violet. Indications seem to justify 

 the theory that there are at least three different kinds of cones, and that these 

 are sensitive to the different wave lengths of light which produce the sensa- 

 tions of red, blue, and green color. According to this theory, the sensation 

 of white results when all kinds of cones are stimulated equally, and inter- 

 mediate colors result when two kinds of cone cells are stimulated unequally. 



Defects of Vision. The most common defects of the human eye are near- 

 sightedness (myopia), farsightedness (hypermetropia), and astigmatism. In 

 the normal eye, the retina is the proper distance behind the cornea and lens 

 for the light rays to come to a focus or point on the fovea. In the nearsighted 

 eye, the eyeball is too long and the light rays converge in front of the retina 

 and are diverging when they reach it; thus they produce a blurred image 

 (Fig. 17.18). In the farsighted eye, the eyeball is too short and the retina 

 too close to the lens; the rays come in contact with the retina before they 

 converge. With age the lens loses its elasticity, does not become more convex 

 in accommodation, and the eye is chronically farsighted. 



Astigmatism, meaning "off the points," results from irregularities in the 

 curvature of the cornea or the lens. In one plane the rays are brought in focus 

 at different points from that of the rays in another plane. On the oculist's 

 chart the upright lines may look clear and black, while horizontal ones look 

 blurred and gray. Astigmatism is so common that this appearance on the chart 

 is familiar to almost anyone whose eyesight has been tested. 



FARSIGHTED EYE 



Corrected by 

 Light rays ^,^-— «>^ convex lens ji ^^-'»w 



::.. Blurred I I T~ ^-^^^^ Sharp 



image LJ — X^^^^^^^^ image 



Corrected by 

 Light rays ^^*«i>^— Vv concave 



Blurred 



Sharp 



'"^age 'v^rC^ZI^^--^ / image 



NEARSIGHTED EYE 



Fig. 17.18. Diagrams of some common defects of the eye. Nearsighted eye, 

 with elongated eyeball and rays brought to a focus in front of the retina. Far- 

 sighted eye, with shortened eyeball and light rays in focus behind the retina. 



