99 BULLETIN 581, U. S. DEPARTMENT OF AGRICULTURE. 
Observe each field as to the presence or absence of mold filaments and note the 
result as positive or negative. Examine at least 50 fields, prepared from 2 or more 
mounts. No field should be considered positive unless the aggregate length of the 
filaments present exceeds approximately one-sixth of the diameter of the. field: 
Calculate the proportion of positive fields from the results of the examination of all 
the observed fields and report as percentage of fields containing mold filaments. 
511 YEASTS AND SPORES (TENTATIVE). 
Fill a graduated cylinder with water to the 20 cc mark, and then add the sample 
till the level of the mixture reaches the 30 cc mark. Close the graduate, or pour the 
contents into an Erlenmeyer flask, and shake the mixture vigorously 15 to 20 seconds. 
To facilitate thorough mixing the mixture should not fill more than thtee-fourths of 
the container in which the shaking is performed. For tomato sauce or pastes or 
products running very high in the number of organisms, or of heavy consistency, 
80 cc of water should be used with 10 cc or 10 grams of the sample. In the case of 
exceptionally thick or dry pastes it may be necessary to make an even greater dilution. 
Pour the mixture into a beaker. Thoroughly clean the Thoma-Zeiss counting cell 
so as to give good Newton’s rings. Stir thoroughly the contents of the beaker with 
a scalpel or knife blade, and then, after allowing to stand 3 to 5 seconds, remove a small 
drop and place upon the central disk of the Thoma-Zeiss counting cell and cover 
immediately with the cover glass, observing the same precautions in mounting the 
sample as given under 50. Allow the slide to stand not less than 10 minutes before 
beginning to make the count. Make the count with a magnification of about 180 
to obtain which the following combinations, 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 8 of the blocks, each of which contains 
25 of the small ruled squares). The total number thus obtained equals the number 
of organisms in 1/60 cmm if a dilution of 1 part of the sample with 2 parts of water 
isused. Ifa dilution of 1 part of the sample with 8 parts of water is used the number 
must be multipied by 3. In making the counts the analyst should avoid counting 
an organism twice when it rests on a boundary line between 2 adjacent squares. 
521 BACTERIA (TENTATIVE). 
Estimate the bacteria from the mounted sample used in 51, but allow the sample 
to stand not less than 15 minutes after mounting before counting. Employ a magni- 
fication 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 Spencer compensating ocular 
with a tube length of 190, or their equivalents. Count and record the number of 
bacteria in a small area consisting of 5 of the small-sized squares. Move the slide to 
another portion of the field and count the number on another similar area. Count 5 
such areas, preferably 1 from near each corner of the ruled portion of the slide and 1 
from near the center. Determine the average number of bacteria per area and mul- 
tiply by 2,400,000, which gives the number of bacteria per cc. Jia dilution of 1 part 
of the sample with 8 parts of water instead of 1 part of the sample with 2 parts 
of water is used in making up the sample, then the total count obtained as above 
must be multiplied by 7,200,000. Omit the micrococci type of bacteria in making 
the count. 
1 These numbers refer to the sections as given in the Journal of the Association of Official Chemists, 
November, 1916. 
2 Comment by authors: The organisms counted as “yeasts and spores” are the yeast cells, and yeast 
and mold spores, not bacteria spores. 
