MICROSCOPY 13 



in spiritus aetheris nitrosi, dry and polish with fine linen or paper. No matter 

 how packed, slides and covers cleaned in bulk for future work always collect 

 some dirt before they are required. To avoid unnecessary repetition, slides 

 and cover glasses are placed in jars containing spiritus aetheris nitrosi and dried 

 and polished immediately before use. 



When bacteria are to be examined unstained and particularly when motility 

 or agglutination is to be observed, the best method is to make a hanging-drop 

 preparation. For this purpose slides are made with a concave depression in 

 the center of one side. A drop of the fluid to be examined is placed on the center 

 of a cover glass, the cover glass is inverted and placed over the cavity of the slide 



FIG. 2. HANGING-DROP PREPARATION. 



so that the drop of fluid hanging on it is suspended in the depression on the 

 slide, without touching the slide at any point. To prevent the slide from moving 

 and to exclude air several tiny drops of immersion oil or vaselin are placed on the 

 slide near the concave depression, before placing the cover glass upon it. It is 

 important that the drop of fluid be on the center of the cover glass, suspended 

 in the concave depression without touching the slide, and that the slide be kept 

 level, neither tilting it before placing on the stage of the microscope nor while it 



FIG. 3. STEWART'S COVER-GLASS FORCEPS 



Method of holding cover-glass and applying the stain. Convenient for blood work, and 

 absolutely necessary for bacteriolcgic work; useful also in sputum examination, etc. The 

 lower instrument shows a cover-glass with stain in position. If the cover-glass be grasped 

 with the film or spread side toward the side of the forceps that has the circular bend intended 

 to fit the thumb, shown at A, there will be no danger of losing trace of the side containing the 

 spread, as this side of the forceps is practically always upward. 



is there. Oblique and not too bright illumination is best for the study of hang- 

 ing-drops. 



Bacteria are most commonly examined after staining and must first be fixed to 

 the slide or cover glass to prevent subsequent washing from carrying them away. 



If it is fluid one wishes to examine,, as when the bacteria are in bouillon, 

 milk, etc., a small drop is placed on a perfectly clean cover glass (which has been 

 flamed and allowed to cool) and spread in a thin even film with a platinum loop. 

 Sputum, pus and similar viscid substances are more readily spread in a thin 

 even film by placing a drop on the center of a slide or cover glass then dropping 

 a second slide or cover glass on top of it and allowing the material to spread out 

 between them, then slide the glasses apart. 



When dried substances and bacteria removed from solid culture media are 

 to be examined, a drop of sterile distilled water is placed on a slide or cover glass, 



