Accommodationofthe Eye

Accommodation of the Eye, the mechanism by which the images of objects at varying distances are brought to a focus on the retina. Helmholtz has demonstrated that this is effected by the contraction of the ciliary muscle, which, by influencing the tension of the suspensory ligament of the lens, admits of alteration in the convexity of the anterior lens' surface. In the theory of vision, it is that power which the normal eye possesses of adjusting itself to see objects at different distances. The distance from the lens in the eye to the retina is practically constant. Hence, if the eye were incapable of accommodation, only objects at one definite distance would produce clear images on the retina. But the curvature of the oystalline lens in the normal eye may be so varied by muscular alteration of its anterior surface that objects may be clearly seen at all distances beyond five or six inches. The range of vision is said to extend from six inches to infinity; thus a star may be seen as clearly as one of these letters. With old age the accommodating power diminishes, and the eye has to be assisted by the use of spectacles. This defect is, however, quite distinct from that of long sightedness, in which case the range is abnormal, though the accommodation for that range may be perfect.