34 TEXT-BOOK OF BACTERIOLOGY. 



moist covering. These examples show the occurrence of bacteria 

 on liquid and solid media, and the rules for the treatment and ex- 

 amination of both are as follows: To examine the micro-organ- 

 isms in the bouillon in their uncolored state, the simplest way is to 

 take a platinum wire, bent at the point, remove an}- foreign matter 

 that may adhere to it by heating it to redness in the flame, wait 

 a few moments until it has sufficiently cooled, then dip it into the 

 flask and endeavor to draw up a little of the liquid. The portion 

 thus obtained is spread on a thin cover-glass, and this process is 

 repeated (meanwhile heating the wire each time) till a sufficient 

 quantity of the liquid is placed on the cover-glass. Then turn the 

 latter over and lay it on the slide, so that the fluid to be examined 

 lies between the cover-glass and the slide. The quantity of fluid 

 should be sufficient to form an even capillary layer, without air bub- 

 bles or dry spots, yet it should not extend beyond the edges of the 

 thin glass, still less overflow the surface of it. Proceed in a similar 

 manner with the bacteria that have grown on a solid nourishing 

 substance, remembering, however, that for the purpose of examining 

 them they must first be put into a liquid medium. Put some dis- 

 tilled water on a cover-glass as before, and bring into this a small 

 quantity of bacteria which are taken from the surface of the po- 

 tato with a platinum wire that has been previously heated. All 

 the further steps are identical with those already described. 



Even with liquids containing bacteria, it is often well to place 

 some distilled water on the cover-glass, in order to dilute them be- 

 fore examination. The number of micro-organisms in nourishing 

 solutions is often so extremely great that a certain degree of di- 

 lution is necessary before they can be examined successfully. When 

 such preparations are brought under the microscope, it is best to 

 at once employ the strongest magnifying power. 



Put a drop of immersion oil on the cover-glass, screw down the 

 lens with the coarse adjustment, till it enters the oil, and then pro- 

 ceed to the fine adjustment. Of course the examination must be 

 made with the diaphragm, since the specimen is unstained, and a 

 " structure picture " is desired ; an opening about the size of a pea, 

 with tolerably good light, will be found the most suitable. 



In examining such a preparation the bacteria will be recognized 

 without difficulty. They move across the field, some knocking 

 against and rolling over each other, others swimming slowly, or 

 even lying quite still for a few moments. But this kind of exam- 

 ination has, nevertheless, its very great disadvantages. 



By the pressure of the cover-glass on the slide, inequalities are 

 continually being caused in the layer between them; at the free 



