ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 217 



(6) Miscellaneous. 



Enumeration of Bacteria.* — E. Glynn, M. Powell, A. A. Rees, 

 and G. L. Cox give the results of a large number of observations upon 

 the counting of bacterial vaccines by the Wright, the hsematocytometer, 

 and the plate culture method. The cytometer method recommended 

 by the writers is as follows : two stock solutions are prepared, one of 

 saturated thionin blue (Griibler) in absolute alcohol, the other of 1 p.c. 

 pure carbolic acid in tap-water. These are mixed in the proportion of 

 1 to 40 for most organisms, 1 to 20 for streptococci. The bacterial 

 emulsion is suitably diluted and mixed with the stain. A drop is 

 mounted in a Helber-Glynn counting-cell, that is to say, a cell 0'02 mm. 

 deep and having an extra wide trough around the counting disk. A 

 thin optically plane coverslip, strengthened by a glass collar, is pressed 

 into position, and the drop examined with an immersion lens. The 

 authors from their comparative observations find that the hremato- 

 cytometer method is the best counting method available, and that the 

 cell of - 02 mm. depth is better than that of 0*1 mm. depth ; in the 

 former the optical definition of the bacteria is sharper and the free 

 working distance greater ; in the latter, settlement of bacteria upon 

 the bottom of the cell is much delayed, and so counting is rendered 

 more difficult. The Wright method, based on comparative enumeration 

 of bacteria in an emulsion and cells in normal blood, usually greatly 

 under-estimates the strength of the bacterial emulsion sometimes by 

 100 or even 200 p.c. This is largely due to defective distribution of 

 the cells and bacteria in the film. The plate culture method is cumber- 

 some and tedious. This also under-estimates the number ot bacteria, 

 first, on account of the impossibility of obtaining a homogeneous emul- 

 sion ; second, because all cultures contain a large number of dead or 

 moribund organisms incapable of forming colonies. Gravimetric 

 methods of estimating bacterial vaccines will be considered in a further 

 paper. 



G. H. Macalistei't recommends that organisms in a bacterial vaccine 

 be counted directly in a Helber cell with dark-ground illumination. 

 The suspension is diluted with deci normal hydrochloric acid, and a drop 

 is mounted and examined. The Microscope is fitted with a Zeiss 

 compensating ocular 18 and a dry 7 mm. objective Zeiss C. The eye- 

 piece carries a grating micrometer, and the tube-length is so adjusted 

 that four eye-piece squares fit one of those on the Thoma-Zeiss rulings. 

 The condenser is Beck's old type of Abbe, carrying on the centre of the 

 under surface of its lower lens a disk of black paper to cut off central 

 rays of light. The acid diluent causes the bacteria to settle on the glass 

 surfaces and become immobilised. They can then be counted in the 

 two optical planes, and by focusing from one to the other, and observing 

 the fine-adjustment graduations, the cell depth may be checked. Com- 

 parative observations show that this method gives more constant results, 

 takes less time and causes much less eye-fatigue than any of the other 

 methods in use. Further work is proceeding in order to find the effects 



* Jouru. Path. andBact., xviii. (1914) pp. 379-400. 

 t Journ. Path, and Bact., xviii. (1914) pp.441 -2. 



