METHODS FOR MICROSCOPIC STUDY OF BACTERIA 197 
the agar block, to fill the angles between the sides of the block and the 
cover-slip. This seal hardens at once, preventing slipping of the 
block. Place the preparation in the incubator again for five or ten 
minutes, to dry the agar-agar seal. Invert this preparation over a 
moist chamber and seal the cover-slip in place with white wax or 
paraffin. Vaseline softens too readily at 37° C, allowing shifting of 
the cover-slip. The preparation may then be examined at leisure. ^ 
Fig. 11. — Warm stage, electrically heated, for the cultivation of bacteria. 
C. Dark-field Illumination and Ultramicroscopic Examination. 2— Parti- 
cles less than 140 X 10*^ mm. in diameter are usually not visible under 
the microscope, even with the highest magnifications, but with the 
ultramicroscope, particles as small as 4 X 10'' mm. may be detected.^ 
For the study of very minute particles in suspension, the ultramicro- 
scope of Siedentoff and Zsigmondy^ has been used, but the dark-field 
illumination apparatus of Reichert,^ a much simpler device, readily 
adjusted to any microscope, has largely supplanted it for bacterial 
examinations. With the Reichert apparatus the flagella of bacteria 
and other structures of low-refracti\'e index may be observed. Tre- 
ponema pallidum in fresh smears from lesions is readily seen with the 
dark-field illuminating apparatus. 
1 A light, detachable, electrically heated warm-stage incubator, manufactured by 
the Chicago Surgical and Electrical Company according to specifications furnished by 
the writer is very satisfactory for this purpose. Bacteria may be maintained constantly 
at any desired temperature between that of the room and 45° C. for several days, and 
observed continuously without difficulty. If the warm-stage incubator is attached to 
a graduated mechanical stage, many individual bacteria may be observed in the same 
preparation by recording their respective positions as indicated on the graduated recti- 
linear stage verniers. 
2 Literature to 1907, Ztschr. f. Chemie u. Industrie d. Kolloide, 1907, 1, Nos. 6 and 9. 
3 PhUlip: Phys. Chem., 1923, 2d ed., p. 194. 
* Ztschr. f. wissenschaftl. Mikroskopie, 1909, 26, 391. 
6 Miinchen. med. Wchnschr., 1906, 53, 2130; Hyg. Rund., 1907, 17, 1121; Centralbl. 
f. Bakteriol., orig., 1909, 51, 14. 
