Clean the special Howard cell so that Newton’s rings are produced
between the slide and the cover-glass. Remove the cover and place, by
means of a knife blade or scalpel, a small drop of the sample upon the
central disk; spread the drop evenly over the disk and cover with the
cover-glass so as to give an even spread to the material. It is of the
utmost importance that the drop be mixed thoroughly and spread evenly;
otherwise the insoluble matter, and consequently the molds, are most
abundant at the center of the drop. Squeezing out of the more liquid
portions around the margin must be avoided. In a satisfactory mount
Newton’s rings should be apparent when finally mounted and none of
the liquid should be drawn across the moat and under the cover-glass.
Place the slide under the microscope and examine with a magnification
of about 90 diameters and with such adjustment that each field of
view covers 1.5 sq. mm. This area is of vital importance and may be
obtained by adjusting the draw-tube in such a way that the diameter of
the field becomes 1.382 mm. as determined by measurement with a stage
micrometer.[7] A 16 mm. Zeiss apochromatic objective with a Zeiss X6
compensating ocular or a Spencer 16 mm. apochromatic objective with a
Spencer X10 compensating ocular, or their equivalents, shall be used
to obtain this magnification. Under these conditions the amount of
liquid examined is .15 cmm. per field. 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 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.
[7] Comment by authors: Obviously after the proper draw-tube length has
been secured that adjustment should be noted and always used in making
mold counts.