REFRACTION IN THE EYE. . 695 



however, the adjustment is effected by increasing or diminishing the curva- 

 tures 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 

 crystalline 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 remedied for distant ob- 

 jects, by placing concave lenses before the eyes, by which the rays falling 

 upon the crystalline are diverged. The opposite condition, in which the 

 antero-posterior diameter is too short (hypermetropia), is such that the rays 

 are brought to a focus behind the retina. This is corrected by converging 

 the rays of incidence, by placing convex lenses before the eyes. In old age 

 the crystalline lens becomes flattened, its elasticity is diminished and the 

 power of accommodation is lessened ; conditions which also tend to bring the 

 rays to a focus behind the retina. This condition is called presbyopia. To 

 render near vision as in reading distinct, objects are placed farther from 

 the eye than under normal conditions. The defect may be remedied, as in 

 hypermetropia, by placing convex lenses before the eyes, by which the rays 

 are converged before they fall upon the crystalline lens. 



The mechanism of accommodation will be fully considered in connection 

 with the physiology of the crystalline lens ; and at present it is sufficient to 

 state that in looking at distant objects, the rays as they fall upon the lens 

 are nearly parallel. The lens is then in repose, or " indolent." It is only 

 when an effort is made to see near objects distinctly, that the agents of ac- 

 commodation are called into action ; and then, very slight changes in the 

 curvature of the lens are sufficient to bring the rays* to a focus exactly on the 

 visual surface of the retina. 



Spherical, Monochromatic 'Aberration. In a convex lens in which the 

 surfaces are segments of a sphere, the rays of light from any object are not 

 converged to a uniform focus, and the production of an absolutely distinct 

 image is impossible. For example, if the crystalline lens had regular curva- 

 tures, the rays refracted by its peripheral portion would be brought to a focus 

 in front of the retina ; the focus of the rays converged by the lens near its 

 centre would be behind the retina ; a few, only, of the rays would have their 

 focus at the retina itself ; and as a consequence, the image would appear 

 confused. This is illustrated in imperfectly corrected lenses, and is called 

 spherical aberration. It is also called monochromatic aberration, because it 

 is to be distinguished from an aberration which involves decomposition of 

 light into the colors of the spectrum. If an object be examined under the 

 microscope with an imperfectly corrected objective, it is evident that the field 

 of view is not uniform, and that there is a different focal adjustment for the 

 central and the peripheral portions of the lens. In the construction of 



