Long Sight. This is a disease of age when the eye becomes too flat to converge rays of light to the distance of the optic nerve; but the light, as is shewn in the engraving, arrives at a focus beyond the nerve, and gives a confused picture at the nerve ; nevertheless, if the object is carried further off, the conver-gency is then perfect, but the object at the increased distance becomes too small to be discerned distinctly, as for reading, etc, and hence it is, that such persons are called long-sighted. The defect is, however, corrected by means of a convex lens, by which the rays are made convergent before they reach the eye, and the eye, therefore, has less to perform. Fig. 1. explains long sight, and the use of a convex lens ; and tig. 2. short sight, and the use of a concave lens.

Long Sight 183Long Sight 184