[73] A good microscope, with 1/12 oil-immersion objective and Abbe condenser, is necessary for bacteriological work. The new dark-ground condenser made by James Swift and Son, London, is extremely useful for examining living bacteria under high magnifications.
[74] The most recent method is to preserve the slides without cover-glasses: a drop of cedar oil is placed directly on the preparation, and, after examination, carefully washed off with xylol. Care should be taken not to touch the preparation with the fingers.
[75] Masson et Cie, Publishers.
[76] Klein, E., Micro-organisms and Disease (Macmillan), p. 22. See also, Kanthack and Drysdale, Practical Bacteriology (Macmillan 1896), p. 86.
[77] The subject is not so far removed from direct human interest as one might suppose. For many years Metchnikoff, in Paris, has been studying the bacterial flora of the human intestines. His theory is, that old age is caused by the poisonous products of these intestinal bacteria, and he proposes to counteract the effects of these poisonous organisms, by introducing into the system large quantities of lactic acid bacteria, either in the form of tabloids, or in the form of sour milk cultures.
[78] For the demonstration of flagellæ, the material is taken from a culture on solid media—a young agar culture is best. Make a dilution by Soyka’s method (p. [103]), dry and fix on a cover-glass as described, p. [88], then proceed as follows:—
1. Mordant with a mixture of 10 c.c. 20 per cent. tannin solution, 5 c.c. cold saturated ferrous sulphate, 1 c.c. fuchsin or wool-black solution. This solution is put on the preparation, which is then heated for one minute until steam is given off.
2. Wash under the tap, then in alcohol.
3. Stain with anilin-water-fuchsin solution, prepared by dissolving the dye in anilin-water and adding 1 per cent. of a 1 per cent. solution of caustic soda until the liquid begins to go cloudy.
4. Wash off the stain with distilled water.