REFRACTION IN THE EYE. 789 



died for distant objects 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 plac- 

 ing 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 pres- 

 byopia. 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 hypermetro- 

 pia, 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 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 periph- 

 eral 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. For example, in examining an object with an imperfectly-corrected objec- 

 tive under the microscope, 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 optical instruments, this difficulty may be in part corrected 

 if the rays of light be cut off from the periphery of the lens by a diaphragm, which is 

 an opaque screen with a circular perforation allowing the rays to pass to a restricted por- 

 tion of the lens near its centre. The iris corresponds to the diaphragm of optical instru- 

 ments, and it corrects the spherical aberration of the crystalline in part, by eliminating a 

 portion of the rays that would otherwise fall upon its peripheral portion. But this cor- 

 rection is not sufficient for high magnifying powers ; and it is only by the more or less 

 perfect correction of this kind of aberration by other means, that powerful lenses have 

 been rendered available in optics. 



The spherical aberration of lenses which diverge the rays of light is precisely opposite 

 to the aberration of converging lenses. If, therefore, we construct a compound lens, it 

 is possible to fulfil the conditions necessary to the convergence of all the incident rays to 

 a focus on -a uniform plane, so that the image produced behind the lens is not distorted. 

 Given, for example, a double-convex lens, by which the rays are brought to innumerable 

 focal points situated in different planes. The fact that but a few of these focal points are 

 in the plane of the retina renders the image indistinct. If we place behind this convex 

 lens a concave lens, by the action of which the rays are more or less diverged, the ine- 

 quality of the divergence by different portions of the second lens will have the following 

 effect : As the angle of divergence gradually increases from the centre toward the periph- 

 ery, the rays near the periphery, which are most powerfully converged by the convex 

 lens, will be most widely diverged by the peripheral portion of the concave lens; so that, 

 if the opposite curvatures be accurately adjusted, the aberrant rays may be blended. It 

 is evident that, if all of the rays were equally converged by the convex lens and equally 



