its convexity for them. The scientific term for short-sight is myopia . Long-sight, or hypermetropia , signifies that the eyeball is too short or the lens too flat. Fig. 118 a represents the normal condition of a long-sighted eye. When looking at a distant object the eye thickens slightly and brings the focus forward into the retina. But its thickening power in such an eye is very limited, and consequently the rays from a near object focus behind the retina. It is therefore necessary for a long-sighted person to use convex spectacles for reading the newspaper. As seen in Fig. 118 b , the spectacle lens concentrates the rays before they enter the eye, and so does part of the eye's work for it.
Returning for a moment to the diagram of the eye (Fig. 116), we notice a black patch on the retina near the optic nerve. This