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 36° PHYSIOLOGICAL PHYSICS. [Chap, xxvin. Amici, of Modena (1827), there was shown a method for rendering the object-glass of a highly magnifying microscope completely achromatic. The combination consists of three pairs of lenses, each pair being made of a doubly convex lens of crown glass, cemented by means of Canada balsam (whose refractive index is the same as that of crown
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 36° PHYSIOLOGICAL PHYSICS. [Chap, xxvin. Amici, of Modena (1827), there was shown a method for rendering the object-glass of a highly magnifying microscope completely achromatic. The combination consists of three pairs of lenses, each pair being made of a doubly convex lens of crown glass, cemented by means of Canada balsam (whose refractive index is the same as that of crown glass) to a plano-concave lens. These lenses are placed close to one another, the plane surface being towards the object, and are so arranged that one lens corrects the errors of the other. Fig. 162 shows this combination, in position in the supporting tuoe. With corrected lenses also the angular aperture is increased. The angular aperture is represented by the side part of Fig. 162 by the angle bfb'. This is the angle formed by the extreme rays which are able to pass through the system of lenses. Thus in the figure, Fig. 162. - Achro- tne ravs fa /«' are too oblique to matic combina- pass through the three pairs of tion and Angle J i ?i 7 /., / * of Aperture. lenses, but the rays jb fb pass, and it is between them the angle of aperture is contained. It is evident, of course, that the more rays that pass through the system of lenses the better illuminated will the object appear to be, and the fewer the rays the dimmer the object. So that, from this point of view, any method which in- creases the angular aperture, and thus increases the illumination, is an improvement. Yet it is to be noted that the more that oblique rays are caused to pass through the system, the greater is the difficulty of correcting for spherical aberration; and, even when the correction is complete, the narrower is the border-land between clear definition and blurring of the object. Another point remains
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