COVER-SLIP PREPARATIONS 175 



film and allowed to remain twenty to thirty seconds. The 

 slip is then carefully rinsed in water, and without drying 

 is placed bacteria down upon a slide; the excess of water is 

 taken up by covering it with blotting-paper and gently 

 pressing upon it, after which the preparation is ready for 

 examination. 



Another plan sometimes used is to bring the slip upon 

 the slide, bacteria down, without rinsing off the staining- 

 fluid; the excess of fluid is removed with blotting-paper and 

 the preparation is ready for examination with the micro- 

 scope. This method is satisfactory and time-saving, but 

 must always be practiced with care. The staining-fluid 

 should always be filtered before using, to rid it of insoluble 

 particles which might be taken for bacteria. 



If upon examination the preparation prove of particular 

 interest, so that it is desirable to preserve it, then it may be 

 mounted permanently. The drop of immersion oil is to 

 be removed from the surface of the slip with blotting-paper, 

 and the slip loosened, or rather floated, from the slide by 

 allowing water to flow around its edges. It is then taken 

 up with forceps, carefully deprived of the water adhering to 

 it by means of blotting-paper, and allowed to dry. When 

 dry it is mounted in xylol-Canada-balsam by placing a 

 small drop of the balsam upon the surface of the film, and 

 then inverting the slip upon a clean glass slide. It is some- 

 times desirable to have the balsam harden quickly, and a 

 method that is commonly employed to induce this is as 

 follows : the slide, held by one of its ends between the fingers, 

 is warmed over a gas-flame until quite hot; a drop of balsam 

 is then placed on the center of it, and it is again warmed; 

 the cover-slip is then placed in position, and when the bal- 

 sam is evenly distributed the temperature is rapidly reduced 



