Methods of Determination of Pathogenic 31icrobes. 105 



pig, etc. With these germs we can, at most, only 

 attenuate the color of the tissue elements with the 

 aim of bringing more distinctly into view the deeper 

 color of the bacteria. Such is the principle of a pro- 

 cess of single staining which has been applied with 

 slight variations by different micro-biologists, and 

 which will be further referred to later. 



Patient researches have placed at our command 

 another method, called double staining. This method 

 furnishes excellent preparations and renders great 

 service in the study of the distribution of microbes in 

 the tissues. Double coloration is obtained by differ- 

 ent means, of which the choice is not indifferent, for 

 the same microbe does not take the stain by all of 

 these processes. The manner in which microbes be- 

 have toward the methods of double staining is even 

 utilized for characterizing them. 



The principle of double staining is as follows: 

 1st, the whole preparation is stained uniformly; 2d, 

 all the elements other than bacteria are decolorized 

 whilst the latter retain, on the contrary, the color 

 which was originally imparted to them ; 3d, the 

 parts decolorized by the preceding operation are 

 stained with a T3olor which stands out distinctly from 

 that of the microbe. Thus, the latter being violet, 

 the contrast color will be red or brown; if, on the 

 contrary, the microbes are stained red, the back- 

 ground of the preparation should be stained blue or 

 green. 



We may have to examine liquids, or solid tissues. 

 In this last case it is sometimes necessary to make a 

 previous examination of the organic pulp and study 

 it later in thin sections. We shall therefore consider. 



