80 SPECIAL SENSES. 



only seen distinctly when very near the eye. This defect may 

 be remedied for distant objects by placing concave lenses be- 

 fore 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 con- 

 vex 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 hy- 

 permetropia, by placing convex lenses before the eyes, by 

 which the rays are converged before they fall upon the crys- 

 talline lens. 



The mechanism of accommodation will be fully considered 

 in connection with the physiology of the crystalline lens ; 

 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 accommodation 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 Aberration. In a convex lens, with its surfaces 

 consisting of portions of a perfect 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 we suppose the crystalline lens to present regular 

 curvatures, 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 



