YEASTS AND SPORES 521 
Place the slide under the microscope and examine with a magnifi- 
cation of about 90 diameters and with such adjustment that each field 
of view represents approximately 1.5 sq. mm. of area on the mount.* 
This area is of vital importance and may be obtained by adjusting 
the draw tube to the proper length as determined by actual measure- 
ment of the field, a 16 mm. Zeiss apochromatic objective with a Zeiss 
X6 compensating ocular or a Spencer 16 mm. apochromatic objective 
with a Spencer X10 ocular, or their equivalents, being used to obtain 
the proper magnification. 
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 two or more mounts. No field should be considered pos- 
itive unless the aggregate length of the filaments present exceeds approx- 
imately one-sixth of the diameter of the field. Calculate the propor- 
tion of positive fields from the results of the examination of all the 
observed fields and report as percentage of fields containing mold 
filaments. 
Yeasts and Spores (Tentative). Fill a graduated cylinder with 
water to the 20 c.c. 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 
fifteen to twenty seconds. To facilitate thorough mixing the mixture 
should not fill more than three-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 c.c. 
of water should be used with 10 c.c. or 10 gms. of the sample. In the 
case of exceptionally thick or dry pastes 1t 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 three to five seconds, remove a small drop and place 
upon the central disk of the Thoma-Zeiss counting cell and cover imme- 
diately with the cover glass, observing the same precautions in mounting 
the sample as given above. Allow the slide to stand not less than 
ten minutes before beginning to make the count. Make the count 
* Comment by the authors. In order to have an area of 1.5 sq. mm. the diameter 
of the microscopic field should be 1.882 mm. This is determined by using a stage 
micrometer and adjusting the length of the microscope draw tube. Obviously after 
the proper draw tube length has been secured the adjustment should be noted and 
always used in making the mold counts. 
