28 ESSENTIALS OF BACTERIOLOOY. 



sary to change the objective on the same field, and this insures 

 a great steadiness of the object. 



Great cleanliness is needed in all bacteriological methods ; but 

 nowhere more so than in the microscopical examination. 



The cover-glass should be very carefully washed in alcohol, 

 and dried with a soft linen rag. To remove the stains on the 

 cover-glasses that have been used, they should be soaked in 

 hydrochloric acid or placed in a 6 per cent, aqueous solution of 

 potassium bichromate with 6 per cent, of strong sulphuric acid, 

 washed in water, and kept in absolute alcohol. 



Examination of Unstained Bacteria. As the coloring of bac- 



FIG. 9. 



Platinum Needles. 



teria kills them and changes their shape to some extent, it is pre- 

 ferable to examine them when possible in their natural state. 



We obtain the bacteria for examination, either from liquid or 

 solid media. 



From Liquids. With a long platinum needle, the end of which 

 is bent into a loop, we obtain a small drop from the liquid con- 

 taining the bacteria, and place it on a cover-glass or slide ; 

 careful that no bubbles remain. 



Sterilize Instruments. Eight here we might say that it is 

 best to accustom one's self to passing all instruments, needles, 

 etc., through the flame before and after each procedure ; it in- 

 sures safety ; and once in the habit, it will be done automati- 

 cally. 



From Solid Media. With a straight-pointed platinum needle, 

 a small speck of the medium is taken and rubbed upon a glass 



