NORMAL HJEMATOLOGY 153 



each slide. After removing them from the fixing solution, the speci- 

 mens must be air-dried under a bell jar until all the solution is com- 

 pletely evaporated. 



D. Staining Blood Films. 



There are many stains that may be used to stain the blood cor- 

 puscles. The choice of stain depends largely upon the purpose of 

 the staining. For ordinary histological purposes eosin and hsema- 

 toxylin are the best. 



Technique (1). Fix the blood film in alcohol fifteen minutes, dry, 

 and stain with 1 per cent, aqueous eosin in 50 per cent, alcohol for 

 two minutes, and then counterstain with Delafield's hsematoxylin for 

 a half-minute. Wash in water, allow to air-dry, and then mount in 

 balsam. But for finer work, when parasites are suspected, as the 

 plasmodium malarise, or when a differential count of the white cells 

 is desired, eosin and methylene blue are necessary. 



Technique (2). Just the same as the above, except that a 10 per 

 cent, methylene blue in 50 per cent, alcohol is used instead of the 

 hsematoxylin. 



Technique (3). The other stain mentioned, Ehrlich's triacid, con- 

 tains both acid and basic stains, and stains all structures, differen- 

 tiating each. In special diseases of the blood this is the best stain, 

 for among its other advantages it differentiates the nucleus of a red 

 cell from that of a white cell. 



All blood films must be fixed carefully by heat, preferably in 

 the oven. Allow the stain to remain on the glass for four or 

 five minutes; then wash, air-dry, and mount in balsam as usual. 

 The stain must be of the best quality and accurately prepared. 

 Then the staining depends largely upon the baking. An under- 

 heated or overheated specimen will not stain well ; the first will appear 

 too red from the acid fuchsin, and the latter will be a pale-lemon 

 color from the orange G. stain. 



E. Differential Counting of the Cells. 



Appliances. Blood films stained with eosin and methylene blue 

 or triacid stain; microscope, with one-eighth or one-twelfth oil- 

 immersion objective and a mechanical stage. 



Technique. The stained blood film should have a space at least 

 one-half inch square in its center in which the corpuscles do not over- 

 lap each other. Place the film in focus with the microscope and 

 begin at the upper left-hand corner of the specimen. Count all 

 the various kinds of white cells, and keep a record of the number 

 of each kind counted. Next count all the red cells in the field, and 

 keep a record of the number and also of any peculiar forms and their 



