'impression' cover- glass preparations 87 



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 ia 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. 



Staining of Spores. — The spores of microbes differ from 

 the fully-formed organisms in the resisting power they offer 

 to staining solutions. When ordinary cover-glass prepara- 

 tions, stained in the usual way, are made of some organ- 

 isms — say, for instance, of Bacillus anthracis or Bacillus 

 megatherium — bright unstained spots are sometimes seen, 

 which may be isolated or in the middle or ends of the 

 organisms. These are the spores which have resisted the 

 colouring matter of the stain. All unstained spots in pre- 

 parations are not necessarily spores, as many causes may 



