f the analyzing prism, H, which it fits as well,
but which must never be touched, as the adjustment of the instrument
would be seriously disturbed by moving it. With the key on the proper
nipple it is turned one way or the other until the field is neutral.
Unless the deviation of the zero be greater than 0.5 deg., it will not be
necessary to use the key, but only to note the amount of the
deviation, and for this purpose the observer must not be content with
a single setting, but must perform the operation five or six times,
and take the mean of these different readings. If one or more of the
readings show a deviation of more than 0.3 deg. from the general average,
they should be rejected as incorrect. Between each observation the eye
should be allowed 10 to 20 seconds of rest.

The "setting" of the zero having been performed as above, the
determination of the accurate adjustment of the instrument by means of
the "control" quartz plates is proceeded with. Three such plates will
be furnished with each polariscope, which have "sugar values"
respectively approximating 80 deg., 90 deg., and 100 deg.. These values may vary
with the temperature, and tables are furnished with them which give
their exact value at different temperatures, from 10 deg. to 35 deg. C.

One of these plates is placed in the instrument, and the field
observed; it will be seen that the uniform appearance of the field is
changed. The milled head is turned to the right until the exact point
of neutrality is re-established, just as described above in setting
the zero. The scale is read, the observation repeated, the reading
taken again, and so on until five or six readings have been made. The
average is taken, readings being rejected which show a divergence of
more than 0.3, and the result corrected for the deviation of the zero
point, if any was found, the deviation being added if it was to the
left, and subtracted if to the right. If the adjustment of the
instrument be correct, the result should be the value of the control
plate used, as ascertained from the table, for the temperature of 20 deg..
Each of the three plates is read in the instrument in this way. A
variation of 0.3 from the established values may be allowed for errors
of observation, temperature, etc., but in the hands of a careful
observer a deviation greater than this with one of the three plates,
after a careful setting of the zero, shows that the instrument is not
accurately adjusted.