6/0 SPECIAL SENSES 



made to revolve so rapidly that the impressions made by the two colors 

 are blended, the resulting impression is of white. 



Refraction by Lenses, A ray of light is an imaginary pencil so 

 small as to present but a single line ; and the light admitted to the in- 

 terior of the eye by the pupil is supposed to consist of an infinite num- 

 ber of such rays. In studying the physiology of vision, it is important 

 to recognize the laws of refraction of rays by transparent bodies bounded 

 by curved surfaces, with particular reference to the action of the crystal- 

 line lens. 



The action of a double-convex lens, like the crystalline, in the refrac- 

 tion of light, may be readily understood by an application of the well- 

 known laws of refraction by prisms. A ray of light falling on the side 

 of a prism at an angle is deviated toward a line perpendicular to the sur- 

 face of the prism. As the ray passes from the prism to the air, it is 

 again refracted, but the deviation is then from the perpendicular of the 

 second surface of the prism. In passing through a prism, therefore, 

 the pencil of light is bent, or refracted, toward the base. 



A circle is equivalent to a polygon with an infinite number of sides. 

 A regular double-convex lens is a transparent body bounded by seg- 

 ments of a sphere. Theoretically, a double-convex lens may be assumed 

 to be composed of an infinite number of sections of prisms (Fig. 174, I), 

 or to make the comparison with prisms more striking, though less accu- 

 rate, the lens may be assumed to be composed of prisms, Fig. 174, II 

 (Weinhold). 



If these prisms or sections of prisms are infinitely small, so that the 

 surface of each receives but a single infinitely small pencil of light, these 

 pencils will be refracted toward the bases of the prisms, and different 

 rays of light from all points of an object may be brought to an infinite 

 number of foci, all these foci, for a plane object, being in the same 

 plane. If the number of sections is equal on every side of the centre of 

 the lens, the bases looking toward the axis of the lens, the rays of light 

 will cross at a certain point and the image will be inverted. This is 

 illustrated in Fig. 174, which represents a section of a lens theoretically 

 dissected into six sections of prisms. 



If the lens A B (Fig. 174) is assumed to be free from what is known 

 as spherical aberration, the rays from the point C will be refracted and 

 brought to a focus at the point D. In the same way the rays from E 

 will be brought to a focus at F, the two sets of rays crossing before they 

 reach their focal points. The same is true for all the rays from every 

 point in the image C E, which strike the lens at an angle, but the ray G 

 H, which is perpendicular to the lens, is not deviated. The rays of light 

 are refracted in this way by the cornea and by the crystalline lens. The 



