. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap. XXIX.] THE OPHTHALMOMETER, 395 the one compensating for the other. The iris also aids in diminishing the aberration. The ophthalmometer.—It may be well before concluding this chapter to describe briefly the principle on which this instrument is constructed. It was devised by Helmholtz for the purpose of measuring the size of the images reflected from the surface of the cornea or lens. Knowing the size of the images and the dist
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap. XXIX.] THE OPHTHALMOMETER, 395 the one compensating for the other. The iris also aids in diminishing the aberration. The ophthalmometer.—It may be well before concluding this chapter to describe briefly the principle on which this instrument is constructed. It was devised by Helmholtz for the purpose of measuring the size of the images reflected from the surface of the cornea or lens. Knowing the size of the images and the distance of the object from the reflecting surface, the radius of curvature of the surface can be calcu- lated. The ophthalmometer consists of a tube in which are placed one above another two similar plates of glass with parallel faces. The glass plates revolve on a vertical axis common to both, but, on turning the 7 O screw, the plates revolve in opposite directions. Now we have seen that rays of light falling perpendicularly on a plate with parallel faces will pass straight through without deviation. If the rays fall ob- liquely they will undergo deviation, but will issue from the plate in a direction parallel to that in which they fell upon the glass. One end of the tube T is directed towards the object to be observed, and in the other end is an eye-piece formed of two achromatic lenses, through * o which the observer looks. The principle of the instrument is illus- trated in Fig. 177. In the figure to the left hand, A represents an object from which rays are reflected to the ophthalmometer. Suppose the plates not to have been revolved and that the reflected rays fall perpendicularly upon the plates, they will pass straight through in the. Fig. 177.—The Oph- Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the o
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