ESSAYS ON BAC^TERIOLOGY. 129 



bacteria. In a sc(^re of ways these reactions may be 

 used in the staining and identification of disease 

 germs. It is fortimate that this is true, for thus only 

 is made easy and of everyday value the identification 

 of certain microbes the. recognition of which would 

 otherwise require the more difficult and tedious proc- 

 esses of culture and inoculation. 



For the examination of bacterial specimens rather 

 high powers of the microscope, and therefore also 

 good illumination, are necessary. The latter is se- 

 cured chiefly by condensing lenses placed between the 

 mirror and the object. Immersion objectives have, 

 over dry objectives, the double advantage of increas- 

 ing illumination and diminishing the evils of refrac- 

 tion. Clearness and distinctness of the picture is as 

 important as magnification. That microscope is of 

 little value which shows "men as trees walking." 

 Medium-power lenses may usually suffice for ordinary 

 purposes, provided their definition be good. But for 

 the highest class of work homogeneous immersion ob- 

 jectives, with strong and adjustable illuminating ap- 

 paratus, are necessary. 



Inoculation experiments upon animals serve several 

 purposes. It is by this means that the truth of the 

 germ theory of disease has been finally and fully 

 proven. Inoculation experiments are the final steps 

 in deciding for or against the specific causative agency 

 of a particular germ. They afford, in otherwise 

 doubtful or difficult cases, most valuable diagnostic in- 



