424 SUMMARY OF CUKHEXT EESEAECEIES liELATLXG TO 



expeditious, occupying only fifteen to twenty minutes from the outset of 

 the operations to the commencement of the counting. 



After the concentration of the precipitate, the zinc hydroxide is 

 redissolved by addition of ammonium chloride, and a iarije drop of 

 alkaline methylen-blue or clear carbol fnchsin is added"" as stain. 

 The volume is made up exactly to 1, 2, or S c.cm. as desired, and, after 

 mixing, a number of counts are made. For counting, one of the ruled 

 cell-slides, known as cytometers, may be used. The Thoma-Zeiss 

 h^naocytometer was found to give good results. In this the ruled 

 portion has a capacity of O'l cubic mm., and this is ruled into twenty 

 rows of squares of twenty squares each. The author has olitained as 

 good or better results with a cell made by evenly grinding a circular 

 depression into a fairly thick Microscope slide, about 10 mm. in 

 diameter and exactly O'l mm. deep ; the depth may be measured by 

 means of the micrometer head of the Microscope. A slight notch filed 

 in one side allows any excess of fluid to escape. In using it a large 

 drop of the well-mixed concentrate is placed in the cell and covered by 

 firmly pressing down a perfectly plain cover-glass, and the bacteria iii 

 the whole field are counted. A Sedgwick-Eafter micrometer, ruled in 

 squares and placed in the eyepiece, greatly facilitates counting Avhen 

 there are many organisms. The proper regulation of the light 'coming 

 from the condenser is highly important for seeing the bacteria clearly, 

 and a dark-ground illuminator is also sometimes used. A magnifica- 

 tion of 400 to 500 diameters is quite suflacient. The capacity of each 

 standard Microscope field is determined once for all, from the depth of 

 the cell and the diameter of the field as measured bv a stage 

 micrometer. An experienced observer will count a standard field in two 

 to sixty seconds, according to the number of bacteria. The number of 

 fields to be counted depends on the density of the bacteria and the 

 uniformity of their distribution ; usually from 10 to 50 should be 

 counted, and the average multiplied up properly according to the 

 capacity of the standard field and the volume of concentrate. Tlie 

 following figures represent counts of fields for the same liquid : — 2-i, 2 s, 

 55, 130, 16, 85, 1G3, 44, 27, 18 ; average 59. Variations are chiefly due 

 to the occurrence of clumps of bacteria. 



The accompanying table shows the results of direct counts obtained 

 with various types of water compared with the results found by means 

 of agar and gelatin plates : — 



Water. 



River 



Filtered 



Reservoir 

 Ditch, turbid 

 Well, shallow 

 Well, deep 



Agar plates. 



800 



650 



9000 



12 



18 



1300 



]5,000 



12 



7 



Gelatin plates. 



650 



550 



liquefied 



10 



12 



1400 



liquefied 



8 



Direct. 



2100 



900 



16,000 



25 



30 



2000 



100,000 



15 



30 



