The instrument employed is known as the half shadow apparatus of
Schmidt and Haensch. It is shown in the following cut.
[Illustration]
The tube N contains the illuminating system of lenses and is placed
next to the lamp; the polarizing prism is at O, and the analyzing
prism at H. The quartz wedge compensating system is contained in the
portions of the tube marked F, E, G, and is controlled by the milled
head M. The tube J carries a small telescope, through which the field
of the instrument is viewed, and just above is the reading tube K,
which is provided with a mirror and magnifying lens for reading the
scale.
The tube containing the sugar solution is shown in position in the
trough between the two ends of the instrument. In using the instrument
the lamp is placed at a distance of at least 200 mm. from the end; the
observer seats himself at the opposite end in such a manner as to
bring his eye in line with the tube J. The telescope is moved in or
out until the proper focus is secured, so as to give a clearly defined
image, when the field of the instrument will appear as a round,
luminous disk, divided into two halves by a vertical line passing
through the center, and darker on one half of the disk than on the
other. If the observer, still looking through the telescope, will now
grasp the milled head M and rotate it, first one way and then the
other, he will find that the appearance of the field changes, and at a
certain point the dark half becomes light, and the light half dark. By
rotating the milled head delicately backward and forward over this
point he will be able to find the exact position of the quartz wedge
operated by it, in which the field is neutral, or of the same
intensity of light on both halves.
[Illustration]
The three different appearances presented by the field are best shown
in the above diagram. With the milled head set at the point which
gives the appearance of the middle disk as shown, the eye of the
observer is raised to the reading tube, K, and the position of the
scale is noted. It will be seen that the scale proper is attached to
the quartz wedge, which is moved by the milled head, and attached to
the other quartz wedge is a small scale called a vernier which is
fixed, and which serves for the exact determination of the movable
scale with reference to it. On each side of the zero line of the
vernier a space corresponding to nine divisions of the movable scale
is divided into ten equal parts. By this device the fractional part of
a degree indicated by the position of the zero line is ascertained in
tenths; it is only necessary to count from zero, until a line is found
which makes a continuous line with one on the movable scale.
With the neutral field as indicated above, the zero of the movable
scale should correspond closely with the zero of the vernier unless
the zero point is out of adjustment.
If the observer desires to secure an exact adjustment of the zero of
the scale, or in any case if the latter deviates more than one-half of
a degree, the zero lines are made to coincide by moving the milled
head and securing a neutral field at this point by means of the small
key which comes with the instrument, and which fits into a nipple on
the left hand side of F, the fixed quartz wedge of the compensating
system. This nipple must not be confounded with a similar nipple on
the right hand side of 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°, 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° 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°, 90°, and 100°. These values may vary
with the temperature, and tables are furnished with them which give
their exact value at different temperatures, from 10° to 35° 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°.
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.
The complete verification of the accurate adjustment of the
polariscope by means of three control plates, as given above, should
be employed whenever it is set up for the first time by the officer
using it, whenever it has sustained any serious shock or injury, and
whenever it has been transported from one place to another. It should
also be done at least once a week while the instrument is in active
use.
After the complete verification has been performed as described,
further checking of the instrument is done by means of one control
plate alone, the one approximating 90°, and the setting of the zero
point is dispensed with, the indication of the scale for sugar
solutions being corrected by the amount of deviation shown in the
reading of the 90° control plate from its established value as
ascertained from the table, at the temperature of the room.
For example: A sugar solution polarizes 80.5; the control plate just
before had given a polarization of 91.4, the temperature of the room
during both observations being 25° C. According to the table the value
of the control plate at 25° C. is 91.7; the reading is, therefore, 0.3
too low, and 0.3 is added to the reading of the sugar solution, making
the corrected result 80.8. The temperature of the room should be
ascertained from a standardized thermometer placed close to the
instrument and in such a position as to be subject to the same
conditions.