File:An introduction to the study of the Diatomaceae (1893) (14596464029).jpg

Identifier: introductiontost00mill (find matches)
Title: An introduction to the study of the Diatomaceae
Year: 1893 (1890s)
Authors: Mills, Frederick William Deby, Julien, 1827-1895
Subjects: Diatoms
Publisher: London : Iliffe Washington : Microscopical Pub. Co.
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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tting which can be raised or lowered (in order to focus theAbbe illuminator which it carries), or instantly turned aside.See fig. 1. The student may rely upon any of the followingmakers stands, which- possess a substage :—Mr. C. Baker,Messrs. R. & J. Beck, H. Crouch, Ltd., Powell & Lealand,J. Swift & Son, and W. Watson & Sons. If, however, a really first-class stand is required, theauthor has a preference for Messrs. W. Watson & Sons Van Heurck. The Fig. 2 gives a good representation ofone of these stands. As to objectives it is difficult to advise. A good one-sixthinch, and one inch are the most useful, and will answer mostpurposes, but for the most minute observations oil immersionlenses of great numerical aperture are necessary. It is never advisable to use a higher power objective than aone-twelfth, as the amount of resolution displayed dependsupon the numerical aperture, and not upon the magnifyingpower of the lens. Also penetration (or depth of focus, as it
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FIG. 2. STUDY OF THE DIATOMACEyE. 55 is sometimes termed) is, and mathematically must be, in theinverse ratio of the aperture, and varies at a rapidly in-creasing rate according to the focal length, the rate beingone upon the square of the focus. The two homogeneous oil immersions which the authorprefers are Powell & Lealands apochromatic ^ of 1*4numerical aperture, and Reicherts semi-apochromatic -^ of1-25 N.A., both of these objectives will stand a cone of lightfilling f of their back lenses, without breaking down, thelatter one giving a remarkably pure image for a cheap lens. When working with a i or -^ inch objective a substageilluminator or some form of condenser becomes necessary.A half-inch objective screwed into an adapter fitting thesubstage will answer this purpose; the objective being,of course, inverted. The ordinary Abbe illuminators, costing from 17s. to £1,are cheap and useful lenses, as they can be used with oilimmersion objectives by connecting the top of the ill

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