. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. I30 BACTERIA IN RELATION TO PLANT DISEASES. slow movement, a swing-out condenser (fig. i2o), two substage iris diaphragms, and various other conveniences. For example, the screw-heads, detennining the cross and sidewise movement of the section, are on the same axis and may be reached and moved without changing the position of one's arm. The apochromatic objectives are the only ones recommended for bacteriological work. They cost more than achromatic objectives, but expense is a minor con- sideration. In hot, moist climates
. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. I30 BACTERIA IN RELATION TO PLANT DISEASES. slow movement, a swing-out condenser (fig. i2o), two substage iris diaphragms, and various other conveniences. For example, the screw-heads, detennining the cross and sidewise movement of the section, are on the same axis and may be reached and moved without changing the position of one's arm. The apochromatic objectives are the only ones recommended for bacteriological work. They cost more than achromatic objectives, but expense is a minor con- sideration. In hot, moist climates the older apochromatic objectives of Zeiss fre- quently became clouded, but those made in recent years have given the writer no trouble in the latitude of Washington. They yield sharp images even with high eye-pieces. Of course, compensating oculars must be used with the apochromatic objectives. It is de- ' ~~^ sirable to have the whole series of ob- jectives and e)'e- pieces, but if one is limited for means, very good work can be done with two objectives and three oculars, viz, object- ives i6 mm. and 3 mm. n. a., and compensating oculars 4, 6, and 12. The newer forms of the Abbe camera furnished by Zeiss (fig. 121) leave little to be desired in the wa)' of a drawing camera. PHOTOGRAPHY AND PHOTO- MICROGRAPHY. For permanent records nothing equals photography. It constitutes, therefore, a very important special part of laboratory work, and every student of pathology should make a knowledge of this subject part of his education. Some of the following suggestions will be useful to beginners. The Zeiss Double-Protar lenses, series VII^-, are the best all round photographic lenses made by that finn, and are excelled by none made by any firm. The back or front lens is usually as good as the combination. Excellent photographic lenses are also made by Voigtlaender and by Goerz. Zeiss photographic lenses may be *Fic. 120.—Swing-out condenser and other substage arrangements on Z
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