BACTERIA, PHAGE, AND VIRUS 197 



examination. Many body fluids contain cells or organisms and may 

 be examined without further treatment. Serous fluids are useful 

 mounting materials either as liquids or after clotting. When the 

 optical path difference is not optimal for good visibility the refractive 

 index of the mounting medium may be changed (Section 3.2 of Chapter 

 IV) or other contrasts and types of diffraction plates tried. Table V.l 

 gives refractive indices of some specimens (see also Table IV.l). 



Mounts of living organisms and wet mounts in general should be 

 sealed with petroleum jelly, paraffin, or other suitable material to reduce 

 movements from convection currents, to prevent evaporation and the 

 consequent necessity of periodically adding water, and to hold the cover 

 glass in place. When the aqueous preparation covers only the center 

 three-fourths of the area, paraffin oil may be run imder the cover to 

 seal evaporation. Some immersion oils are sufficiently non-toxic 

 and are preferable to paraffin oil as the index and dispersion are corrected 

 for microscopy. Oxygen will dissolve and be carried through such a 

 thin paraffin layer in adequate amounts for some microorganisms. 

 Various types of mounts, cells, and compressors have been described 

 in the earlier popular books on microscopy. These might well be 

 revived for use with phase microscopy (Beale, 1870; Carpenter, 1901; 

 Hogg, 1886; Quekett, 1852; or other editions). 



3. MICROORGANISMS 



3.1. Bacteria, phage, and virus 



Bacteria should be examined in wet mounts (Fig. V.l). Heat 

 fixation should be avoided whenever possible, because it shrinks and 

 distorts the cells. The larger bacteria may be studied with the 

 0.2A±0.25X diffraction plates, although the 0.14A — 0.25X may be found 

 more useful than the 0.2A — 0.25X plate. The smallest organisms re- 

 quire greater contrast, as do the internal details of the cell, and the 

 0.07A±0.25X plates are preferable. Bright contrast is usually chosen 

 for locating and counting organisms, and dark contrast for measure- 

 ment. The smaller structural details are more easily seen with bright 

 contrast although the dark contrast resembles the appearance of 



Fig. V.l. Bacteria. A-D, Bacillus mycoides, 1500 X: A, brightfield. B, dark- 

 contrast (B — ) phase. C, dark-contrast (A — ) phase. D, bright -contrast phase. 

 E-J, Mycobacterium lephrae, 4000 X: E, brightfield. F, dark-contrast phase. 

 G-J, bright-contrast phase. K, giant .spirochete, 1600 X. Bright-contrast phase. 

 Lr-0, pneumococci showing capsules, 3000 X: M, brightfield. .V, dark-contrast 

 phase. L, 0, bright-contrast phase. P, Q, Bacillus cereus, 2500 X : P, dark-contrast 



phase. Q, bright-contrast phase. 



