BACTERIA 



243 



the warmed cover-glass preparation and kept 2 minutes. 

 The excess of stain was blotted off, and the cover-slip 

 immersed in 25% nitric or sulphuric acid The retention 

 of the red colour by Bacillus tuberculosis under this 

 treatment is characteristic. The preparation was washed 

 for 1 to 1J minutes in tap water, and counterstained by 

 immersion in a saturated watery solution of methylene 

 blue, or in Loffler's methylene blue. 



Micrococcus in Pus (Plate 33.E). Stained by Gram's 

 method (page 266), by which the bacteria are stained but 

 the pus cells are decolourised. This was followed by 

 immersion in Bismark brown until the pus cells took a 

 light brown colour, but owing to the use of a yellow- 

 green screen the brown comes out light and the purple- 

 stained bacteria very dark. 



Living Yeasts, Moulds, Bacteria, etc. (Plate 36). 

 Research in connection with the life history and func- 

 tions of various species of yeasts and other micro- 

 organisms forms a very important part of the scientific 

 control in Brewing and the Fermentation industries, 

 and a true photomicrographic record of the appear- 

 ance of the organisms met with is of great value. 

 Bacteria are almost invariably prepared for photo- 

 micrography by some fixing and staining process, but the 

 application of heat or reagents involves a certain amount 

 of risk of alteration in the appearance of any micro- 

 organism, and this is particularly the case with yeasts 

 and moulds, which many workers consequently prefer 

 to photograph in their living state. There are, however, 

 many difficulties to be encountered arising from their 

 small size, their transparency and slight differentiation 

 from the surrounding field, and from their motion in the 

 mounting fluid. They are usually mounted in a drop of 

 water or of the culture medium in which they are growing, 

 with a cover -glass lightly pressed down on it. There is 

 not much risk of damage to the smaller organisms if the 



