160 BACTERIOLOGICAL DIAGNOSIS. 



above we had to add a rider marked 5 to the standard 

 to bring about an exact match, the percentage amount 

 of haemoglobin in the blood would be 55. 



It is an advantage to place cell and standards side by 

 side rather than one above the other, for the upper and 

 lower portions of the retina differ in sensitiveness to 

 colour, whilst the sides do not. 



PREPARATION OF FILMS FOR STAINING. 



The processes to be described are the same whether 

 we are making films for the purpose of investigating the 

 nature of the corpuscles and cells contained, or for the 

 detection of parasites. If, in the latter case, we are 

 searching for the malaria parasite very good films are 

 necessary, but if we are only looking for bacteria we 

 may use films that are quite useless for a study of the 

 blood itself. 



We shall describe two methods, one with cover- 

 glasses and one with slides and cigarette papers. 



I. METHOD WITH COVER-GLASSES. 



Requisites. i. Perfectly clean cover-glasses. These 

 must have been cleaned with nitric acid, washed, and 

 then soaked in ammonia, then washed in absolute 

 alcohol and kept in a mixture of equal parts of absolute 

 alcohol and ether. Immediately before they are re- 

 quired for use they must be removed with a clean 

 pair of forceps and dried with an old and soft handker- 

 chief. They may then be passed through the flame. 



