SPECTACLES

are needed in certain cases of defective vision to a ist the 7 fn focu ing the rays of light on to the retina. In “ far-sighted ne ,”” a common defect of advancing age, the rays of light from near objects are not sufficiently refracted by the ordinary a dope of the eye, and tend to converge to a focus beern the retina. In such cases convex naes of suitable curvature remedy the defect. The eyes of short-sighted persons have too much convergent power, and naturally an image of the object looked at is formed in front of the retina. Concave lenses enable the eye in such cases to focus the image on the retina Itself. [s. 285]