THE MICROSCOPE AND MICROSCOPIC METHODS 37 



Smear Preparations for Staining.— The examination of 

 bacteria with the microscope is carried out to a very large extent 

 by means of smears made upon thin slips of glass. Such slips 

 of glass are generally called cover-glasses. It is best to obtain the 

 kind sold by dealers as No. i, ^i inch squares. 



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

 ture 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, or perhaps preferably in alcohol containing 3 per cent 

 of hydrochloric acid. 



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 ^eeded 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 difl&culty 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 2oo°C. for half anjiour. 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. 



