EXAMINATION AND STAINING OF BACTERIA 43 



The preparation may have a drop of cedar oil placed directly 

 upon it and so be examined under the oil immersion lens ; if, how- 

 ever, the slide is to be preserved for future examinations it is better 

 to first place a small drop of Canada balsam on the film and cover 

 it with a coverslip. 



Blood Films. A blood film is prepared by placing a drop of 

 blood near the end of a clean slide and the edge of a second slide 

 is then lowered on to the drop 

 at an angle to the first slide. 

 By capillarity the blood spreads 

 itself along the edge of the 



Second Slide, which is gently FIG. 14Meth^of making Blood 



stroked over the surface of the 



first, leaving a film the width and thickness of which depends on 



the angle at which the slide was held (Fig. 14). 



Another method employed is to place a drop of blood between 

 two coverslips and then draw them apart. If the red blood 

 corpuscles are to be examined, fixing should not be effected by heat ; 

 the slide may be placed in a mixture of equal parts of alcohol and 

 ether for half an hour and then washed and dried in the air, or it 

 may be placed in a saturated solution of corrosive sublimate for 

 two or three minutes then washed well in running water and dried. 

 The method of staining depends upon the object for which the 

 examination is to be made. 



THE STAINING OF BACTERIA 



Staining Principles. Generally speaking, bacteria react to 

 stains in a manner similar to the nuclei of animal cells ; the pro- 

 cess is to be regarded as a chemical combination between the dye 

 and the cell substance rather than merely a mechanical saturation. 

 Certain organisms stain readily, others only with great difficulty ; 

 the easily stained forms require immersing but a few minutes in 

 a watery dye while those which do not stain so readily require a 

 longer time and in some cases the addition of heat or a mordant. 

 The tubercle bacillus belongs to the latter class. Spores and 



