522 FOOD PRESERVATION 
with a magnification of about 180 to obtain which the following com- 
binations, or their equivalents, should be employed: 8 mm. Zeiss 
apochromatic objective with X6 Zeiss compensating ocular, or an 8 mm. 
Spencer apochromatic objective with X10 Spencer compensating ocular 
with draw tube not extended. 
Count the number of yeasts and spores * on one-half of the ruled 
squares on the disk (this amounts to counting the number in eight of the 
blocks, each of which contains twenty-five of the small ruled squares). 
The total number thus obtained equals the number of organisms in 
1/60 c.mm. if a dilution of one part of the sample with two parts of 
water is used. If a dilution of one part of the sample with eight parts 
of water is used the number must be multiplied by three. In making 
the counts the analyst should avoid counting an organism twice when 
it rests on a boundary line between two adjacent squares. 
Bacteria (Tentative). Estimate the bacteria from the mounted 
sample used above, but allow the sample to stand not less than fifteen 
minutes after mounting before counting.t Employ a magnification of 
about 500, which may be obtained by the use of an 8 mm. Zeiss apochro- 
matic objective with an X18 Zeiss compensating ocular with draw tube 
not extended, or an 8 mm. Spencer apochromatic objective with an X20 
Spertcer compensating ocular with a tube length of 190, or their equiva- 
lents. Count and record the number of bacteria in a small area consist- 
ing of five of the small-sized squares. Move the slide to another por- 
tion of the field and count the number on another similar area. Count 
five such areas, preferably one from near each corner of the ruled portion 
of the slide and one from near the center. Determine the average num- 
ber of bacteria per area and multiply by 2,400,000, which gives the num- 
ber of bacteria per cubic centimeter. If a dilution of one part of the 
sample with eight parts of water instead of one part of the sample with 
two parts of water is used in making up the sample, then the total count 
oLtained as above must be multiplied by 7,200,000. Omit the micro- 
cocci type of bacteria in making the count. 
Explanation of Calculations. Fig. 80 has been prepared to make 
somewhat clearer the explanation of the areas denoting the yeast and 
spore and bacterial counts. The light lines in the figure show the 
arrangement of rulings on the entire slide. The squares (A, A, etc.), 
and rectangles (B, B, etc.) designated in the figure by the heavy lines 
indicate the portions used for the yeast and spore and for the bacterial 
* Comment by authors. The organisms counted as ‘ yeasts and spores” are 
the yeast cells, and yeast and mold spores, not bacteria spores. 
T The estimation of yeast, molds and bacteria are made from the same mount. 
