. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. PHYSIOLOGICAL PHYSICS. [Chap. xxvn. true for spherical mirrors that all the reflected rays meet in one point. Similarly, it is not absolutely true for lenses that the refracted rays meet in one point, though it becomes more nearly true the smaller the aperture (page 314) of the lens. The rays from the circumference of the lens are focussed nearer to the lens than rays from more central parts of the lens. The result is, that when the
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. PHYSIOLOGICAL PHYSICS. [Chap. xxvn. true for spherical mirrors that all the reflected rays meet in one point. Similarly, it is not absolutely true for lenses that the refracted rays meet in one point, though it becomes more nearly true the smaller the aperture (page 314) of the lens. The rays from the circumference of the lens are focussed nearer to the lens than rays from more central parts of the lens. The result is, that when the centre of the image is well defined the circumference is blurred, and vice versa, because the focal points for the outer and inner rays do not correspond. This is shown in Fig. 157, where A is the focus for the outer rays a c, and JB is the focus for the central rays b. At the position B the centre is in focus, but not Fig. 157.—S herical AVer- ,u . ,, mi . , ration. the circumterence. Ihis aber- ration is easily rectified by cut- ting off the external rays. In front of the lens, there- fore, a diaphragm or stop is usually placed with an opening in the centre. In photography the diaphragm is of the utmost consequence. Every photographer prefers to have such illumination as will permit him to use a diaphragm with a very small opening, since this adds to the definition and sharpness of his image. It, of course, at the same time diminishes the amount of light. In the chapter on the eye it is noted how the iris acts as diaphragm, and varies in the size of its pupil with the amount of light. Spherical aberration can also be corrected by a combination of lenses of suitable 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 original M'Gregor-Robertson, J. (Joseph), 1858-1925. Philadelphia, H. C. Lea's son & c
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