ain brought up over the end by adding a few more drops. The
cover glass being in position, the tube is closed by screwing on the
cap. The greatest care must be observed in screwing down the caps that
they do not press too tightly upon the cover glasses; by such pressure
the glasses themselves may become optically active, and cause
erroneous readings when placed in the instrument. It should therefore
be ascertained that the rubber washers are in position over the cover
glasses, and the caps should be screwed on lightly. It must also be
remembered that a cover glass, once compressed, may part with its
acquired optical activity very slowly, and some time must be allowed
to elapse before it is used again.

The polariscopic reading may now be taken, an observation on the 90 deg.
control plate having been made immediately before as previously
described. Then without altering the position of the instrument
relative to the light, or changing the character of the latter in any
way, the tube filled with the sugar solution is substituted for the
control plate. The telescope is adjusted, if necessary, so as to give
a sharply defined field, which must appear round and clear. (This
condition must be fulfilled before the observation is performed, as it
is essential to accuracy.) The milled head is turned until the neutral
point is found, and the reading is taken exactly as previously
described, the operation repeated five or six times, the average taken
with the rejection of aberrant readings, the average figure corrected
for the deviation shown by the control observation from the sugar
value of the control plate at the temperature of observation as given
in the table, and the result taken as the polarization of the sugar.
When a series of successive polarizations is made under the same
conditions as regards temperature, position of the instrument with
relation to the high intensity, of the light, etc., the control
observation need not be made before each polarization, one such
observation being sufficient for the entire series. The control must
be repeated at least once an hour, however, and oftener when the
operator has reason to think that any of the factors indicated above
have been altered, for any such alteration of conditions may change
the zero point of the instrument.

In the polarization of the quartz plates, as also in the polarization
of very white sugars, difficulty may be experienced in obtaining a
complete corresponde