46 EXPERIMENTAL DAIRY BACTERIOLOGY 



mount. If it is then desired to make a permanent mount, 

 allow the preparation to dry, when the cover will fall from the 

 slide when the latter is gently tapped. The cover can then be 

 mounted in "balsam. Allow the balsam to become dry before 

 examining again (which process takes several days), as other- 

 wise the cover glass is likely to be displaced. Preparations 

 made on the slide may be drained, carefully blotted with 

 filter paper, and allowed to become perfectly dry before ex- 

 amining, as the immersion oil is placed directly on the prep- 

 aration. If it is desired to make a permanent mount of such 

 a preparation, the immersion oil is removed by the use of 

 xylene, a drop of balsam placed on the slide, and a cover 

 glass placed in position. 



When a preparation is properly made from a surface 

 growth on solid media, the bacteria should appear as though 

 suspended in air, i.e. the field of the microscope should be 

 perfectly clear and the bacteria evenly distributed in the 

 field. The common error is the use of too much growth, re- 

 sulting in a thick film in which the bacteria are crowded 

 together and piled up in such a manner as to make a satis- 

 factory study impossible. 



Exercise. Make cover-glass preparations from the cultures fur- 

 nished you, using methylene blue or alkaline methylene blue for 

 preparations from milk, broth, and gelatin cultures, gentian violet 

 for preparations from agar. 



Examination of stained preparations. In the examination 

 of cover-glass preparations the following points should be 

 noted: (1) the form of the bacteria, whether spherical, elon- 

 gated, or spiral; (2) the grouping of the cells, whether iso- 

 lated, in long or short filaments, spherical cells in plane 

 surfaces, in irregular masses, or in packets ; (3) the in- 

 ternal structure of the cell, in both faintly and deeply stained 



