=392. Defects of Vision.=--There are several defects of vision that may be corrected by spectacles or eye-glasses. One of these is "near-sightedness." It is due either to an eyeball that is elongated, or to an eye lens that is too convex, or to both conditions. This condition brings light from distant objects to a focus too soon (as shown in Fig. 389). Only light from near objects will focus upon the retina in such cases. With _normal_ vision light from _distant_ or _near_ objects may be focused without unusual effort upon the retina, see Fig. 390. The remedy for near-sightedness is to use concave lenses which will assist in properly refracting the light so the focus will be formed on the retina (Fig. 391). "Far-sightedness" is the reverse of near-sightedness; the eyeball is either too short, or the lens too flat, or both conditions obtain, so that the light entering the eye is brought to a focus behind the eyeball (Fig. 392). The remedy is convex lenses which will assist in properly converging the light, see Fig. 393. A third defect is called _astigmatism_. This is caused by some irregularity or lack of symmetry in the eye. It is corrected by a _cylindrical_ lens that compensates for this defect of the eye. A diagram similar to Fig. 394 is used as a test for astigmatism. If the lines appear with unequal distinctness, some irregularity of refraction (astigmatism) is indicated. [Illustration: FIG. 390.--The normal eye. The parallel rays _A B_ focus without accommodative effort at _C_.] [Illustration: FIG. 391.--Correction of near-sightedness by concave lens.] [Illustration: FIG. 392.--Far-sightedness or hyperopia. Parallel rays focused behind the retina.] [Illustration: FIG. 393.--Correction of far-sightedness by a convex lens.] [Illustration: FIG. 394.--Test card for astigmatism.] =393. The Photographic Camera.=--This is a light-tight box, provided with a convex lens in front, covering an aperture and a ground glass screen at the back. The distance between the lens and the screen is adjusted until a sharp image is obtained upon the latter, which is then replaced by a sensitive plate or film. The sensitized surface of the plate or film contains a salt of silver which is changed by the action of light. After the plate has been "exposed" to the action of light, it is "developed" by the use of chemicals producing a _negative_ image. From "negative," by the use of sensitized paper, "positive" prints may be secured which resemble the object photographed. [Illustration: FIG. 395.--Diagram of the projecting lantern.]