EXAMINATION OP BACTERIA WITH THE MICROSCOPE. 25 



Cover-glass Preparations.— The study of bacteria with 

 the microscope is for the most part done by means of smears 

 made upon thin cover-glasses. It is best to obtain the kind sold 

 by dealers as No. i, f inch squares. 



The cover-glass may be cleaned best by immersion in a 

 mixture of sulphuric acid and bichromate of potassium solution, 

 and afterward washed thoroughly in distilled water, and finally 

 in alcohol. A stock of clean cover-glasses may be kept in a 

 bottle of alcohol. 



Cleaning Fluid. 



Potassium bichromate 40 grams. 



Water 150 c.c. 



Dissolve the bichromate of potassium in the water, 

 with heat; allow it to cool; then add slowly and 

 with care sulphuric acid, commercial 230 c.c. 



For most purposes it is sufhcient to wash the cover-glass in 

 alcohol containing 3 per cent, of hydrochloric acid. It should 

 then be wiped clean with a piece of linen cloth. Whenever it is 

 taken into the fingers it should be held by the edges, never by 

 the flat surfaces. In spreading bacteria upon it and in all 

 subsequent manipulations, as staining, the cover-glass should 

 be handled with the forceps. It can be used very conveniently 

 in the form of forceps known as the Comet forceps, or in the 

 modification devised by Stewart. Bacteria may be placed upon 

 the cover-glass by allowing the glass to fall upon one of the 

 colonies of bacteria, on a gelatin or agar plate (see page 88), 

 which will adhere to it in part, producing an "impression prepa- 

 ration" (German, Klatschpreparat) . Such a preparation, after 

 drying in the air, is to be fixed by passing it through the flame 

 three times. (See below.) The forceps with which it is 

 handled should be sterilized in the flame. 



Generally bacteria contained in fluids, like sputum, or taken 

 from the surface of a culture, are smeared over the cover-glass 

 by means of the platinum wire or loop, which must be heated to 

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