MICROSCOPICAL EXAMINATION OF BACTERIA. 85 



or fatty partkiles ; on the other hand, micro-organisms remain 

 unaffected by these reagents. Baumgarten demonstrated tubercle 

 bacilh in sections by treating them with potash, which clarified 

 the tissues and brought the bacilli clearly into view. Actinomyces 

 and other vegetable structures will not disappear when sections are 

 immersed in weak hydrochloric acid and mounted in glycer'ine. 



In examining unstained bacteria, it is necessary, in order to 

 obtain the structure picture, that the light entering the microscope 

 should be reduced by employing a small diaphragm, and the sub-stage 

 condenser carefully centred and focussed. To focus an unstained 

 specimen in which only bacteria are present, is often difficult. The 

 slide may be gently raised towards the objective, and the stage 

 may be constructed to enable tliis to be done with the index finger 

 (Mg. 1 6). If on tilting the slide the organisms come into focus it 

 will serve as a guide in working the fine adjustment. Another plan 

 when bacteria are examined in water, is to look for an air-bubble, 

 and then to focus its edge until the bacteria appear in view. 



The simple method of covering the liquid with a cover-glass will 

 not answer for a prolonged examination, as the liquid evaporates and 

 the specimen dries up. To keep living bacteria under observation 

 for any length of time, in order to study their movements or spore- 

 formation, a special slide must be employed (p. 120). 



Stained Bacteria. 



Weigert first pointed out the value of the aniline dyes for 

 staining bacteria, and we are principally indebted to Koch, EhrHch, 

 Gram, and Loffler for many valuable processes. 



The staining of fresh preparations, especially those with no 

 coagulable albumen to fix; them, may be carried out by the method 

 of His. A slide is prepared as already described for the exami- 

 nation of micro-organisms in the fresh state. The reagents are 

 then appUed by placing them with a pipette drop by drop at 

 one margin of the cover-glass, and causing them to flow through 

 the preparation by means of a strip of filter-paper placed at the 

 opposite margin. 



Bab^s recommends another rapid means of examining cultivations. 

 A little of the growth, removed by means of a sterilised platinum 

 hook or small loop, is spread out on a cover-glass into as thin a 

 film as possible : when almost dry, a drop or two of a weak aqueous 

 solution of methyl violet is allowed to fall from a pipette upon the 

 film. The cover-glass with the drop of stain is, after a minute, 



