694 



SPECIAL SENSES. 



parison with prisms more striking, although less accurate, the lens may be 

 assumed to be composed of prisms (Fig. 254, II., Weinhold). 



If these prisms or sections of prisms be infinitely small, so that the sur- 

 face 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 be 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 formed by the lens will be inverted. This is illustrated in Fig. 



II 



FIG. 254. Refraction by convex lenses. 



254, which represents a section of a lens theoretically dissected into six sec- 

 tions of prisms. 



If the lens A B (Fig. 254) be 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 retina is normally at such a distance 

 from the lens that the rays are brought to a focus exactly at its surface. In- 

 asmuch as the rays cross each other before they reach the retina, the image 

 is always inverted. 



Supposing the crystalline lens to be free from spherical and chromatic 

 aberration, the formation of a perfect image depends upon the following con- 

 ditions : 



The object must be at a certain distance from the lens. If the object be 

 too near, the rays, as they strike the lens, are too divergent and are brought 

 to a focus beyond the plane F II D, or behind the retina ; and as a conse- 

 quence the image is confused. In optical instruments the adjustment is 

 made for objects at different distances by moving the lens itself. In the eye, 



