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 hemoglobin 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. MetTHop WITH COVER-GLASSES. 
Requisites—1. 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. 
