THE MICROSCOPE IN MEDICINE AND SANITATION. 117 
be used with a drop of cedar-oil between the lens and 
the cover-glass, or the slide, if no cover-glass be used. 
The Abbé condenser is also a necessity for lighting bac- 
terial preparations. 
The diagnosis of diphtheria is somewhat more com- 
plicated than that of tuberculosis and involves cultural 
as well as microscopical methods. The organism occurs 
at the back of the throat and material for examination 
is obtained by rubbing a sterile swab of cotton over the 
whitish patches which appear on the pharynx and fauces. 
This swab is then passed over a slanting surface of coag- 
ulated blood-serum upon which after incubation for six 
to twelve hours at the body temperature minute pearly 
colonies of the bacilli will appear. A portion of the 
growth is then removed, with a sterile loop of platinum 
wire and spread over a clean cover-slip in a drop of 
water. The cover thus prepared is dried, fixed by pass- 
ing three times through the flame, and stained for ten 
minutes with Loeffler’s methylene blue (30 cc. of a 
saturated alcoholic solution of the dye mixed with 100 cc. 
of a 4/10,000 aqueous solution of caustic potash). After 
washing, drying, and mounting in balsam, the speci- 
men, in cases of diphtheria, will show the very variable 
rods characteristic of the diphtheria bacillus, many of 
them cigar-shaped or club-shaped, and often exhibiting 
the peculiar cross-barred appearance indicated in Fig. 
46. Sometimes these organisms will be seen mixed 
with spherical bacteria arranged in pairs and chains, 
the streptococci. In negative cultures only cocci are 
found, as a rule, although other bacilli, more or less 
