METHODS OF OBSERVING BACTERIA. 147 



Cover-glass Preparations for General Examination. 

 — For bacteriological purposes thin covers (No. 1) are 

 generally required, because thick glasses interfere with 

 the focussing of the oil-immersion lenses. The cover- 

 glasses can never be too clean. It is best to immerse 

 them first in a strong mineral acid, then to wash them in 

 water, then in alcohol, then in ether, and keep them in 

 ether until they are to be used. Except that it some- 

 times cracks, bends, or fuses the edges of the glasses, a 

 better and more convenient method of cleaning them is to 

 wipe them as clean as possible, seize them in fine-pointed 

 forceps, pass them repeatedly through a small Bunsen 

 flame until it becomes greenish yellow, then slowly ele- 

 vate the glasses above the flame, so as to allow them to 

 anneal. This maneuvre removes the organic matter by 

 combustion. It is not expedient to use covers twice for 

 bacteriological work, though if well cleaned they may 

 subsequently be employed for ordinary microscopic ob- 

 jects. 



The material to be examined must be spread in the 

 thinnest possible layer upon the surface of a perfectly 

 clean cover-glass and dried. It must next be fixed to 

 the glass by immersion for twenty-four hours in equal 

 parts of absolute alcohol and ether, or, as is much easier 

 and more rapid, be passed three times through aflame, 

 experience having shown that when drawn through the 

 flame three times the desired effect seems best accom- 

 plished. The Germans recommend that a Bunsen 

 burner or a large alcohol lamp be used, that the arm 

 holding the forceps containing the cover-glass describe 

 a circle a foot in diameter, and that, as each revolution 

 occupies a second of time, the glass be made to pass 

 through the flame from apex to base three times. This 

 is supposed to be exactly the requisite amount of heating. 

 The rule is a good one for the inexperienced. 



Inequality in the size of various flames may make it 

 desirable to have a more accurate rule. Novy l suggests 



1 Laboratory Work in Bacteriology, 1 899. 



