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 preparation. This number is recorded as the denominator of a vulgar fraction of which the numerator is the number recorded for the specific serum. This fraction, expressed as a percentage of unity = the _opsonic index_. IMMUNE BODY. Immune body or amboceptor is the name given to a substance present in the serum of an infected animal that has successfully resisted inoculation with some particular micro-organism, and which possesses the power of linking the complement normally present in the serum to bacteria of the species used as antigen in such a manner that the micro-organisms are rendered innocuous, and ultimately destroyed. The presence of the immune body in the serum can be demonstrated _in vitro_ by the reaction elaborated by Bordet and Gengou, known as the complement fixation test, the existence or the absence of the phenomenon of complement fixation being rendered obvious macroscopically by the absence or presence of hæmolysis on the subsequent addition of "sensitised" red blood corpuscles, (e. g., a mixture of crythrocyte solution and the appropriate hæmolysin--two of the three essentials in the hæmolytic system, _vide_ page 326). _Apparatus Required:_ Sterile pipettes 1 c.c., (graduated in tenths). 16 × 2 cm. test-tubes. 9 × 1 cm. test-tubes. Test-tube racks for each size of test-tube. _Reagents Required:_ Normal saline solution. Erythrocyte solution (human red cells, page 329) = E. Hæmolytic serum (for human cells) = H.S. Complement (fresh guinea-pig serum) = C. Specific serum from inoculated animal, inactivated = S.S. Control pooled serum from normal animals of same species, Inactivated = P.S. _Antigen_ (cultivation upon solid medium of the organism (e. g., B. typhosus) which has already served as antigen in the inoculation of the experimental animal) = A.