MORPHOLOGY OF LEUCOCYTES AND RED CORPUSCLES 259 



the films are placed in a mixture of equal parts of alcohol and 

 ether for at least half an hour. 



This method of fixation is good, but slow. 



IV. Fixation by Formalin. There are several methods by 

 which the fixative action of formalin can be used for blood- 

 work. Of these, the use of a mixture of I part of formalin 

 with 9 parts of absolute alcohol answers perfectly. The films 

 are immersed in this for half a minute, and then washed 

 thoroughly under the tap. 



This method of fixation is both good and rapid. 



STAINING BLOOD-FILMS FOR THE INVESTIGATION OF THEIR 



CELLS. 



There are a great many methods of staining blood-films, 

 and all depend upon the division of stains into two varieties, 

 the acid and the basic. All the stains which are used in this 

 branch of histology are salts, and in some of these salts the 

 acid radicle does the staining, in others the basic. 



Acid stains are those in which the colouring property resides 

 in the acid of the salt. A familiar example is picrate of potash, 

 a yellow stain in which the picric acid is the active ingredient. 

 The acid stains in chief use are eosin, acid fuchsin, and orange 

 G. Substances which stain with an acid stain after suitable 

 exposure to a mixture of an acid and a basic stain are called 

 oxyphile, or, from the frequent use made of eosin as an acid 

 stain, eosinophile. 



Basic stains are those in which the colouring property re- 

 sides in the basic radicle of the salt; they include all the stains 

 which are in use for staining bacteria, and they all colour the 

 nuclei of cells. The most important are methylene blue, 

 methyl green, and toluidin. Ordinary basic fuchsin used in 

 staining the tubercle bacillus belongs to this group, as do 

 hsematoxylin, carmine, etc. 



We shall describe three methods of staining, and these are 

 sufficient for all purposes of diagnosis. They are: (i) 

 Ehrlich's method with his triacid stain; (2) Jenner*s method; 

 and (3) eosin and methylene used separately. Of these, the 

 second method is the simplest, and all that is necessary in the 

 vast majority of cases. The third method is an emergency 

 one, for use when Jenner's stain is not at hand : the first is 



