CHAPTER VII. 



THE STAINING 01 COVBB-GLASS PEEPAEATIONS. 



Unstained bacteria cannot easily be seen even with 

 the best instruments, although, in consequence of their 

 different refractive powers, they stand out faintly from 

 the surrounding medium. The observation of these 

 living unstained bacteria is of course the most impor- 

 tant, but as a rule this difference in refractive power 

 is really too small for the recognition of delicate details 

 in their construction and form, which can be easily 

 seen if their optical appearance is altered. This can 

 be best brought' about by staining — that is, by soak- 

 ing the bacteria thoroughly in some substance which 

 has the power of absorbing a number of colours, and 

 of transmitting light of certain fixed wave lengths. 

 Bacteria are specially suitable for staining, as most of 

 them take up certain aniline dyes with great avidity, 

 and become intensely coloured. 



There are two ways of staining bacteria : either the 

 tissue containing them is cut into thin sections, which 

 are stained in a suitable manner (see following chap- 

 ter) ; or the bacteria are placed on cover-glasses and 



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