788 SPECIAL SENSES. 



the air, it is again refracted, but then the deviation is from the perpendicular of the sec- 

 ond surface of the prism. If we imagine two prisms placed together, as in Fig. 251, the 

 ray A B will be bent toward the perpendicular G B to M. As it passes from the prism, 

 it will be refracted from the perpendicular H M and take the direction M I. Correspond- 

 ing refraction takes place in the ray 1ST O falling upon the lower prism. These two rays 

 will cross each other at the point L. 



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

 A regular double-convex lens is a transparent body bounded by portions of a sphere, and 

 it may be assumed to be composed of an infinite number of prisms. The action of a con- 

 vex lens is to converge the rays of light falling upon different portions of their surface so 

 that they cross at a certain distance behind the lens. If we imagine the lens A B (Fig. 

 252) to be free from spherical aberration, the rays C D and C E, from the point C, will 



FIG. 252. Refraction T>y convex lenses. 



be refracted and brought to a focus at the point F. In the same way. the rays from the 

 point K will be brought to a focus at the point L, the two sets of rays crossing at G. 

 The same is true for all the rays from the object C K, which strike the lens at an angle ; 

 but the ray H I, which is perpendicular to the lens, is not deviated. The line H I is 

 called the axis of the lens. These facts may be applied to the crystalline lens. The rays 

 from an object C K fall upon the lens and are brought to a focus so as to produce the 

 image L F. The retina is supposed to be at such a distance from the lens that the rays 

 are brought to a focus exactly at its surface. Inasmuch as the rays cross each other at 

 the point G, 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 conditions : 



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 L I F, or behind the retina ; and, as a consequence, the image is confused. In 

 optical instruments, the adjustment is made for objects at different distances by moving 

 the lens itself. In the eye, however, the adjustment is effected by increasing or dimin- 

 ishing the curvatures of the lens, so that the rays are always brought to a focus at the 

 visual surface of the retina. The faculty of thus changing the curvatures of the crys- 

 talline lens is called accommodation. This power, however, is restricted within certain 

 well -defined limits. 



In some individuals, the antero-posterior diameter of the eye is too long, and the 

 rays, for most objects, come to a focus before they reach the retina. This defect may be 

 remedied by placing the object very near the eye, so as to increase the divergence of the 

 rays as they strike the crystalline. Such persons are said to be near-sighted (myopic), 

 and objects are seen distinctly only when very near the eye. This defect may be reme- 



