284 PRACTICAL DAIRY BACTERIOLOGY 



i 



bacteria will not stain. Place on the cover-glass enough of one 

 of the staining solutions to cover it completely, and allow the 

 specimen to stand undisturbed for several minutes. The length 

 of time varies with the conditions : usually 2 to 5 minutes is 

 long enough. Wash off the stain with water. The preparation 

 is then ready to study with a microscope. If only a few mo- 

 ments' study is desired, the cover-glass can now be placed on 

 a glass slide, in a drop of water, with the bacteria side down, 

 and studied directly with the microscope. If, however, it is 

 desired to preserve the specimen for future study, dry the cover- 

 glass, then place a drop of Canada balsam on a glass slide, and 

 upon this put the cover-glass, bacteria side down. Study the 

 preparations thus made with a microscope. For this purpose 

 it is necessary to have high magnifying powers. Because of 

 the minuteness of the bacteria, practically nothing can be made 

 out concerning their shapes, with a magnifying power less than 

 about 1,000 diameters, and to obtain this it is common to use a 

 i/12-inch immersion lens. Place the prepared specimen on the 

 microscope stage, and put a drop of immersion oil on top of the 

 cover-glass. Lower the i/12-inch lens into the immersion oil, 

 and then carefully focus upon the bacteria. If the microscope 

 has an Abbe condenser or a diaphragm below, it is best to have 

 this widely open, for the colored bacteria are more easily seen 

 with an open diaphragm. The bacteria are very small, and even 

 with this magnifying power it may sometimes be difficult to 

 make out their shape; but ordinarily their general appearance 

 can easily be made out. Examine in this way all the cultures 

 isolated in No. 14, and sketch. 



*No. 16. Comparison of Yeast and Bacteria. Rub up in a 

 watch glass a bit of a cake of yeast with a little water. Place 

 a drop of the liquid on a slide and examine directly with the 

 microscope, using a I /6-inch objective. Dry a little of the 

 material on a slide and stain exactly as in staining bacteria 

 (No. 15). Study with the immersion lens, comparing the yeast 



