The student's guide to diseases of the eye . Fig. 13.—Virtual erect image formed by a convex lens. of projection of about 12 (30 cm.) is taken in com-paring the magnifying power of different lenses. When a & is at / (Fig. 13) we shall find on trialthat the image a b can be seen well only by bringingthe eye close up to the lens. At a greater distanceonly part of the object will be seen, and this part willbe less brightly lighted, facts of importance in directophthalmoscopic examination (p. 52). Thus in Fig. 14. FlG. 14.—Virtual image; result of observer varying distanceof his eye from the lens.
The student's guide to diseases of the eye . Fig. 13.—Virtual erect image formed by a convex lens. of projection of about 12 (30 cm.) is taken in com-paring the magnifying power of different lenses. When a & is at / (Fig. 13) we shall find on trialthat the image a b can be seen well only by bringingthe eye close up to the lens. At a greater distanceonly part of the object will be seen, and this part willbe less brightly lighted, facts of importance in directophthalmoscopic examination (p. 52). Thus in Fig. 14. FlG. 14.—Virtual image; result of observer varying distanceof his eye from the lens. an observer placed anywhere between the lens andx, receiving rays from every part of a b, will see thewhole image. But if he withdraw to y, his eye willreceive rays only from the central part of a b, andwill therefore not see the ends of the object. It is easily shown by similar constructions thatthe images formed by concave lenses are alwaysvirtual, erect, and diminished, whatever the distanceof the object (Fig. 15). (Compare Fig. 11). OPTICAL OUTLINES 17. The size of the image (whether real or virtual)varies with (1) the focal length of the lens, and (2)the distance of the object from the principal focus.
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