INSPECTION OF COLONIES. 



25 



For the inspection of colonies and of snbcultures in tnbes the best liand-lens 

 known to the writer is the Zeiss aplanat magnifying six times (fig. 25). That magni- 

 fying 10 times is also very useful, but will not reach to the center of an ordinary test 

 tube. Those in apple-tree wood cases are in some respects more convenient than 

 those provided with metallic swing covers (fig. 26). 



The best general work to consult on the morphology of the bacteria is 

 undoiibtedly Migula's System (sec ISibliog., III). 



Pmv.siologv. 



In the description of bacteria we are compelled to make large use of ph\'siolog- 

 ical peculiarities, owing to their \-er\' simi.)le and monotonous morjdiology. Within 

 the limits of the genera now recognized the form differences are so very slight 



Fig. 23.« 



that many bacteria, c. .<,'•., Bacillus coli, />'. cloaca'^ B. suipcstifcr^ B. typhosus^ 

 /)'. tiDiylovorus^ etc., are indistinguishable under the microscope. In mixed cultures, 

 or stained preparations, no one could distinguish one from the other with an)- cer- 

 taintv, and in pure cultures of unknown origin certain identification by means of 

 the microscope would be equally impossible. Nevertheless, these same forms are so 

 widel)' dififerent in their behavior in cultiu'e media, in their pathogenic properties, in 

 their relation to heat, air, antiseptics, etc., that we are certainly warranted in regard- 

 ino- them as distinct species, using the word "species" in its common acceptation. 

 These well-ascertained facts should uot, however, lead one to neglect sliglit differ- 

 ences of form, even when they can be expressed onl}- in fractions of a micron. On 



*Fii'.. 23. Zeiss compensating ocular No. 12 widi screw-filar micrometer. 



