The elements of physiological physics The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology elementsofphysio00mgre Year: 1884 354 PHYSIOLOGICAL PHYSICS. [Chap, xxvin. A converging lens is also frequently employed. The effect on the reflected rays is shown in Fig. 160. The observed eye is represented by A, and a is a point on the retina. The reflected rays passing through the media of the eye would, if permitted, meet at 6, and an image of part of the retina represented by the arrow a would be produce
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The elements of physiological physics The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology elementsofphysio00mgre Year: 1884 354 PHYSIOLOGICAL PHYSICS. [Chap, xxvin. A converging lens is also frequently employed. The effect on the reflected rays is shown in Fig. 160. The observed eye is represented by A, and a is a point on the retina. The reflected rays passing through the media of the eye would, if permitted, meet at 6, and an image of part of the retina represented by the arrow a would be produced, represented by the arrow eb. 1 But the convex lens B refracts the rays still more, and the result is that the rays are focussed at d, nearer to the observed eye than eb. Consequently the n- Fig. 160.—The Ophthalmoscope, with Inverted Image. observer's eye c, placed at the distance for distinct vision, sees an image f, smaller than eb, inverted, and real. Thus the ophthalmoscopic mirror, alone or in con- junction with a concave lens, gives a virtual erect image, considerably magnified. The convex lens gives a real inverted image, and considerably less magnified. The focal length of the convex lens usually employed is about 6 centimetres. With such a lens the extent of magnification is about four times, but with a lens of longer focus it would be increased. There are many forms of ophthalmoscope which it is not necessary to consider here. In general the concave mirror ought to have a focal length of about *—' O 18 centimetres, and the convex lens one of 6 cc. Usually there are also supplied two discs, each of which has, round its circumference, a series of circular j openings, about 8 millimetres in diameter. In one disc, each of these openings is occupied by small con- vex lenses of varying focal length, in the other by