LABORATORY RESEARCH, ETC. 263 



colour is made lighter by -washing with absolute 

 alcohol and oil of cloves, and the preparation is after- 

 wards mounted in balsam. The nuclei of the cells are 

 then of a carmine red, and the bacteria are violet ; the 

 rest of the preparation is of a much paler colour. 



Ehrlich's Method. — We mentioned this method 

 when speaking of the bacillus of tuberculosis. It 

 consists in treating the sections or mounted prepara- 

 tion with a solution of methyl-violet in anUine oil, and 

 the colour is afterwards quickly discharged in nitric 

 acid ; the bacteria alone remain violet. Fuchsin, 

 methylene blue, coccinine, vesuvine, etc., are also em- 

 ployed in various processes for staining bacteria. 



Measurement, Drawings, and Photograplis. — Bac- 

 teria are measured by comparing them with the 

 divisions of the micro-millimetre slide placed on the 

 stage of the microscope over the preparation. The 

 microbes may be drawn without much difficulty by 

 means of the camera lucidar— at least, after a little 

 practice, as their forms are not at all complex. But 

 the results afforded by photography are, as it is plain, 

 very superior. The photographic plate is indeed more 

 sensitive than the eye, and often allows us to see 

 details which had escaped the latter. Koch has given 

 good illustrations of pathogenic bacteria in his book 

 entitled, Beitrage zur Biologie der FJlanzen, voL ii. 

 (1877). 



Methods of Microbe Culture. — The development of 

 microbes may be observed by placing the drop of 



