[Illustration: Fig. 157. 1. Focus of parallel rays. 2. Focus of divergent rays. 3. Focus of divergent rays brought forward by more convex lens.] The use of spectacles to long or short-sighted people is a necessity, and the lenses used vary. The eye has usually the capacity of suiting itself to viewing objects—its accommodation, as it is termed—near or far. But when the forepart of the eye is curved, and cannot adapt itself to distant objects, the person is said to be short-sighted. In long sight the axis of the eyeball is too short, and the focus falls beyond the retina; in short sight it is too long. In the diagrams herewith fig. 159 shows by the dotted lines the position of the retina in long sight, and fig. 160 in short sight, the clear lines showing in each case the perfectly-formed eye. For long sight and old sight the double-convex glass is used, for short sight the double-concave (fig. 162). We know the burning-glass gives us a small image of the sun as it converges the rays to its focus. But lenses will do more than this, and in the PHOTOGRAPHIC CAMERA we find great interest and amusement. [Illustration: Fig. 158.—Diverging rays.] Photography (or writing by light) depends upon the property which certain preparations possess of being blackened by exposure to light while in contact with matter. By an achromatic arrangement of lenses the camera gives us a representation of the desired object Fig. 163 shows the image on the plate, and figs. 164 and 165 the arrangement of lenses. [Illustration: Fig. 159.—Hypermetropia (long sight)..] [Illustration: Fig. 160.—Myopia (short sight).] [Illustration: Fig. 161.—Concave and convex lenses.] [Illustration: Fig. 162.—Lenses for long and short sight.] To Porta, the Neapolitan physician, whose name we have already mentioned more than once, is due the first idea of the Photographic Camera. He found that if light was admitted through a small aperture, objects from which rays reached the hole would be reflected on the wall like a picture. To this fact we are indebted for the CAMERA OBSCURA, which receives the picture upon a plane surface by an arrangement of lenses. In fact, Porta nearly arrived at the Daguerreotype process. He thought he could teach people to draw by following the focussed picture with a crayon, but he could not conquer the aërial perspective. So the camera languished till 1820, when Wedgwood and Sir Humphrey Davy attempted to obtain some views with nitrate of silver, but they became obliterated when exposed to the daylight. [Illustration: Fig. 163.—The Camera.]