13. Tube H, carrying a nicol analyzer and ocular O for defining the
field of vision: This tube is rotated by the radial arm G, carrying the
mirror and reading glass.
=72 Manipulation.=—The lamp having been adjusted, the instrument, in a
dark room, is so directed that the most luminous spot of the flame is
in the line of vision. An observation tube filled with water is placed
in the trough and the zero of the vernier is placed accurately on the
zero of the scale. The even tint of the field of vision is then secured
by adjusting the apparatus by the device number 12.
=73 The Soleil-Ventzke Polariscope.=—A form of polariscope giving a
colored field of vision was in use in this country almost exclusively
until within ten years, and is still largely employed. There are
many forms of tint instruments, but the one almost exclusively used
here is that mentioned. A full description of their construction and
manipulation is given by Tucker.[43] By the introduction of a third
rotating nicol in front of the lens next to the lamp, the sensitive
tint at which the reading is made can be kept at a maximum delicacy.
These instruments are capable of rendering very reliable service,
especially in the hands of those who have a delicate perception of
color. They are inferior, however, to the shadow instruments in
delicacy, and are more trying to the eye of the observer. The shadow
instruments therefore, especially those making use of an ordinary
kerosene lamp, have practically driven the tint polariscopes out of use.
The general arrangement of a tint instrument as modified by Scheibler
is shown in Fig. 37.
[Illustration: FIGURE 37. TINT POLARISCOPE.]
Beginning on the right of the figures, its optical parts are as
follows: A is a nicol which, with the quartz plate B, forms the
apparatus for producing the light rose neutral tint. The proper
degree of rotation of these two parts is secured by means of the
button L attached to the rod carrying the ratchet wheel as shown. The
polarizing nicol is at C, and D is a quartz disk, one-half of which is
right-handed and the other left-handed. At G is another quartz plate
belonging to the analyzing part of the apparatus. This, together with
the fixed quartz wedge F, and the movable quartz wedge E, constitute
the compensating apparatus of the instrument whereby the deviation
produced in the plane of polarized light by the solution in the tube is
restored.
Next to the compensating apparatus is the analyzing nicol which in
this instrument is fixed in a certain place, _viz._, the zero of the
scale. The analyzer and the telescope for observing the field of vision
are carried in the tube HJ. The movable quartz wedge has a scale which
is read with a telescope K, provided with a mirror inclined at an angle
of 45°, just over the scale and serving to illuminate it. The quartz
wedges are also provided with a movement by which the zero point of
the scale can be adjusted. A kerosene lamp with two flat wicks is the
best source of illumination and the instrument should be used in a dark
room and the light of the lamp, save that which passes through the
polariscope, be suppressed by a shade. The sensitive or transition tint
is produced by that position of the regulating apparatus which gives
a field of view of such a nature that a given small movement of the
quartz compensating wedge gives the greatest contrast in color between
the halves of the field of vision. For most eyes a faint rose-purple
tint, as nearly colorless as possible, possesses this quality. A slight
movement of the quartz wedge by means of the screw head M will, with
this tint, produce on one side a faint green and on the other a pink
color, which are in strong contrast. The neutral point is reached by so
adjusting the quartz wedge as to give to both halves of the field the
same faint rose-purple tint.
=74. The Shadow Polariscope for Lamp Light.=—This form of instrument
is now in general use for saccharimetric purposes. It possesses on
the one hand, the advantages of those instruments using monochromatic
light, and on the other, the ease of manipulation possessed by the tint
polariscopes. It differs from the tint instrument in dispensing with
the nicol and quartz plate used to regulate the sensitive tint, and
in having its polarizing nicol peculiarly constructed in harmony with
the optical principles of the jellet-corny prism. The more improved
forms of the apparatus have a double quartz wedge compensation. The
two wedges are of opposite optical properties, and serve to make
the observations more accurate by mutual correction. The optical
arrangement of the different parts of such a polariscope is shown in
the following figure.
The lenses for concentrating the rays of light and rendering them
parallel are contained in the tube N. At O is placed the modified
polarizing nicol. The two compensating quartz wedges are moved by the
milled screw-heads EG. The rest of the optical apparatus is arranged
as described under the tint polariscope. For practical purposes, only
one of the wedges is employed, but for all accurate work the readings
should be made with both wedges and thus every possible source of error
eliminated.
[Illustration: FIGURE 38. DOUBLE COMPENSATING SHADOW POLARISCOPE.]
=75. The Triple Shadow Instrument.=—When properly made, all the
instruments which have been mentioned, are capable of giving accurate
results if used in harmony with the directions given. In the use
of polariscopes having colored fields of vision a delicate sense
of distinguishing between related tints is necessary to good work.
Color-blind observers could not successfully use such apparatus. In
the shadow instruments it is only necessary to distinguish between the
halves of a field of vision unequally illuminated and to reduce this
inequality to zero. A neutral field is thus secured of one intensity
of illumination and of only one color, usually yellow. Such a field of
vision permits of the easy discrimination between the intensity of the
coloration of its two halves, and is consequently not trying to the eye
of the observer, and allows of great accuracy of discrimination. This
field of vision has lately been still further improved by dividing it
into three parts instead of two. An instrument of this kind, Fig. 39,
in use in this laboratory, permits a delicacy of reading not possessed
by any other instrument used for sugar analysis, and approaching that
of the standard Landolt-Lippich apparatus, used by us for research
work and for determining the rotation of quartz plates and testing the
accuracy of other polariscopes.
[Illustration: FIGURE 39. TRIPLE SHADOW POLARISCOPE.]
The triple shadow is secured by interposing in front of the polarizing
nicol two small nicols as indicated in Fig. 40. The end views in
different positions of the polarizer are shown in the lower part of the
diagrams.
[Illustration: FIGURE 40. APPARATUS FOR PRODUCING A TRIPLE SHADOW.]
Instead of the comparison of the intensity of the illumination being
made on the halves of the field of vision it is made by comparing
the segments of the halves with a central band, which also changes
in intensity synchronously with the two segments, but in an opposite
direction.