it "control" or "N.S." (normal serum) and
place in the box by the side of the specific serum preparation.

7. Place the box with the pipettes in the incubator and set the signal
clock to ring at 15 minutes (or start the stop watch).

8. At the expiration of the incubation time remove the pipettes from the
incubator.

9. Cut off the sealed end of the specific serum preparation. Mix its
contents thoroughly as in step 3, and then divide the mixture between
two 3 by 1 slips and carefully spread a blood film (_vide_ page 376) on
each in such a way that only one-half of the surface of each slide is
covered with blood--the free edge of the blood film approximating to the
longitudinal axis of the slide.

Allow films to dry and label the slides with writing diamond.

10. Treat the contents of the control pipette in similar fashion.

11. Select the better film from each pair for fixing and staining.

12. Fixing and staining must be carried out under strictly comparable
conditions, and to this end the slides are best handled by placing in a
glass staining rack which can be lowered in turn into each of a series
of glass troughs containing the various reagents (Fig. 196). Place the
rack in the first trough which contains the alcoholic solution of
Leishman's stain for two minutes to fix.

Transfer to the second trough containing the diluted stain for ten
minutes.

Transfer to the third trough containing distilled water, and holding the
trough over a sink, run in a stream of distilled water until washing is
complete. Remove slides from the rack and dry.

Leishman's stain is the best for routine work for all bacteria other
than B. tuberculosis. Films containing tubercle bacilli must of course
be stained by the Ziehl Neelsen method.

[Illustration: FIG. 196. Glass staining trough for blood films.]

13. Examine specific serum slide microscopically with 1/12 inch oil
immersion. Find the edge of the blood film--along this the bulk of the
leucocytes will be collected. Starting at one end of the film move the
slide slowly across the microscope stage and as each leucocyte comes
into view count and record the number of ingested bacteria. The sum of
the contents of the first 50 consecutive polymorphonuclears that are
encountered is marked down. (The _average_ number of bacilli ingested
per leucocyte = the "_phagocytic index_.")

14. In precisely similar manner enumerate the bacteria present in the
first 50 cells of the control p