In preparing plates, such amounts of the water under examination shall
be planted as will give from 25 to 250 colonies on a plate;[202] and the
aim should be always to have at least two plates giving colonies between
these limits. Where it is possible to obtain plates showing colonies
within these limits, only such plates should be considered in recording
results, except where the same amount of water has been planted in two
or more plates, of which one gives colonies within these limits, while
the others give less than 25 or more than 250. In such case, the result
recorded should be the average of all the plates planted with this
amount of water. Ordinarily it is not desirable to plant more than 1 cc.
of water in a plate; therefore, when the total number of colonies
developing from 1 cc. is less than 25, it is obviously necessary to
record the results as observed, disregarding the general rule given
above.
Counting shall in all cases be done with a lens of 2½ diameter’s
magnification, 3½X. The Engraver’s Lens No. 146 made by the Bausch &
Lomb Optical Company fills the requirements, and is a convenient lens
for the purpose.