88 APPLIED BACTERIOLOGY 



If a permanent preparation is not required, the cover- 

 glass can be examined immediately after washing off the 

 excess of stain by placing on a glass slip, taking care to dry 

 the top surface of the cover-glass before applying the drop 

 of cedar-oil. 



Smear Preparations. — In cases where micro-organisms 

 are found in the blood and tissues of the body, their 

 presence may be demonstrated by making a smear pre- 

 paration. A drop of the blood is spread in a very thin 

 layer over a perfectly clean cover-glass, or it may be 

 brought in contact with the freshly-cut surface of the 

 organ, such as the liver or spleen. Another method is to 

 press the material between two cover-glasses, which are 

 then separated by sliding them apart, thus leaving a thin 

 layer of the material on each cover-glass. This method is 

 particularly applicable to blood and sputum. The cover- 

 glasses are now air-dried and stained, as described under 

 cover-glass preparations. 



' Impression ' Cover-glass Preparations. — These prepara- 

 tions are frequently known as ' contact ' preparations. 

 They are made as follows : A cover - glass, cleaned as 

 already directed, is held with a pair of forceps over a 

 colony (which for this purpose should be a young one, not 

 exceeding 2 millimetres in diameter), and placed with one 

 edge resting on the nutrient surface, in a slanting position ; 

 the cover-glass is then allowed to sink gradually down over 

 the colony, and very gently pressed. The cover-glass is 

 now carefully lifted with a needle, and allowed to dry spon- 

 taneously in the air. The preparation is now ' fixed ' and 

 stained, as described under the preparation of ordinary 

 cover-glass preparations. By this method many very beau- 

 tiful preparations are yielded by a large number of bacteria 

 growing in plate cultures, and which show very clearly the 

 manner of growth and the arrangement of the organisms. 



