166 ATLAS OF BACTERIOLOGY. 



it as the most valuable diagnostic measure, for ex- 

 ample, in deciding the question whether an organism 

 is to be regarded as a cholera vibrio or not. Pfeiffer 

 made the same discovery in regard to bacterium typhi 

 and its allies, and this is corroborated by Dunbar, 

 Sobernheim, Loffler, and Abel. 



It must not be forgotten, however, in opposition to 

 these very interesting and surprising findings that a 

 number of investigators (for example, Hiippe) do 

 not recognize a sharp distinction between resistance 

 and specific immunity, but acknowledge only quanti- 

 tative, not qualitative, differences. At all events, we 

 still have much to learn in this difficult field. 



Technical Appendix. 



The following recommendations and brief descrip- 

 tions furnish all the technical directions which are 

 given in a thorough course of bacteriology. We have 

 given only the most necessary data and those which 

 in our experience have proved most practical. 



I. MICROSCOPICAL EXAMINATION OF BACTERIA. 



1. HINTS ON MICROSCOPICAL TECHNIQUE. 



For bacteriological examinations we use almost ex- 

 clusively the modern microscope with Abbe's illu- 

 minating apparatus, iris diaphragm, a low-power 

 lens, and an oil immersion lens. 



A. Low magnifying power (sixty to one hundred 



times) and narrow diaphragm are used for careful 



examination of plate cultures. For this purpose we 



either raise the cover* and examine the colony from 



*Our plate cultures are always poured into cups. 



