COVER-GLASS PREPARATIONS 51 



case of some organisms, by using hot staining solution, 

 the cover-glass, well covered with the stain, is held by 

 forceps over a low gas-flame until steam just begins to 

 rise from the liquid, when the source of heat is removed* 

 This treatment is repeated at frequent intervals. A 

 better method is to float the cover-glass face downwards 

 upon the staining liquid, which has just previously been 

 heated in a small dish until the steam begins to rise. Great 

 care must be taken not to allow the stain to boil, as this 

 causes a precipitation of colouring matter, which renders 

 the preparation useless. The cover-glass is then well 

 rinsed in running water until no more colouring-matter 

 conies away. The cover-glass is blotted between filter- 

 paper and allowed to dry spontaneously. Sorop workers 

 make their preparations on the slide instead of a cover-glass. 



A small drop of a thick solution of Canada balsam in 

 xylol is placed in the centre of a clean glass microscope 

 slip, and the cover-glass, prepared surface downwards, 

 deposited on the drop of balsam, which then spreads out. 

 The preparation can now be observed by placing a drop 

 of cedar oil on the top of the cover-glass, and examining 

 with the oil-immersion lens. After examination the cedar 

 oil on the cover-glass is carefully absorbed with filter- 

 paper. After a few days the balsam will become hard. 



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, or may be dried and examined in cedar 

 oil or liquid petrolatum. 



Smear Preparations. A cover-glass is 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, leaving a thin layer of 

 material on each. This method is particularly applicable 

 to blood and sputum. The smears are air- dried and 

 stained, as described under cover-glass preparations. 

 (For preparation of blood films see p. 46.) Smears of 

 blood, pus, and organs may be stained by Leishman's or 

 Jenner's methods (see p. 186). 



'Impression' Cover-Glass Preparations. ('Contact* 

 Preparations). A cover-glass is held with a pair of forceps 



