Fig. 387.—The visual angle, AOB is greater at AB than at A´B´.
391. The Visual Angle.—To examine objects carefully we usually bring them as close to the eye as possible, for the nearer to the eye the object is brought, the larger is the visual angle formed by it (see Fig. 387), and the larger is its image upon the retina. The visual angle of an object is the angle at the eye lens between the rays that have come from the ends of the object. Consequently the more distant the object, the smaller is its visual angle. Now if we wish to examine small objects with great care, we frequently find that it is necessary to bring them close to the eye so that they have a visual angle of adequate size. If they must be brought closer than 10 in. a double convex lens is placed in front of the eye. This assists the eye lens in converging the light so that a clear image may be formed when the object is close, say an inch or so from the eye. This is the principle of the magnifying glass used by watch-makers and of the simple microscope. The action of the latter is illustrated by Fig. 388. The convex lens forms a virtual, enlarged image "A´-B´" of the object "A-B" which it observed instead of the object itself.
Fig. 388.—Action of the simple microscope.
Fig. 389.—"Near sightedness", or myopia. Parallel rays come to a focus at F; emerging rays focus at A, the far point.
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.
Fig. 390.—The normal eye. The parallel rays A B focus without accommodative effort at C.
Fig. 391.—Correction of near-sightedness by concave lens.
Fig. 392.—Far-sightedness or hyperopia. Parallel rays focused behind the retina.
