THE MICROSCOPE AND MICROSCOPIC METHODS 37 



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, com- 

 mercial 230 c.c. 



When they are needed for use they should be wiped clean 

 with a piece of linen cloth. As a rule, cover-glasses cleaned in 

 this way still retain a small amount of oily matter on their surfaces, 

 sufficient to prevent the proper spreading of a drop of water. 

 This difficulty may be overcome by passing each glass several 

 times through the flame. It is better, when time permits, to fill 

 an Esmarch dish with clean cover-glasses and then heat them in 

 the oven at 200 C. for half an hour. Cover-glasses treated in this 

 way will allow the droplet of bacterial suspension or other material 

 to spread perfectly. They must be carefully preserved in a 

 covered dish from which they are to be removed only by clean 

 (flamed) forceps. Carelessness in this matter may necessitate 

 recleaning of the whole lot of cover- glasses. 



An ordinary pair of fine forceps may be used to pick up the 

 cover-glass and insert it between the blades of such special forceps 

 as those of Cornet or of Stewart. Perhaps the most convenient 

 style of forceps is that devised by Novy, provided with a clasp. 

 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 no), which will adhere to it in part, produc- 

 ing an " impression preparation" (German, Klatschpreparat). 

 Such a preparation, after drying in the air, is to be fixed by pass- 

 ing 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 

 a red heat before and after the operation. Such preparations 



