=68. Rotation Instruments.=—This instrument has already been described
as one in which the extent of deviation in the plane of polarized light
caused by the intervention of an optically active substance is measured
by rotating one of the nicols about its axis and measuring the degree
of this rotation by a vernier on a graduated arc.
In Germany these instruments are called _polaristrobometers_, and
in France _polarimètrés_. In England and this country the term
_polariscope_ or _polarimeter_ is applied without discrimination to all
kinds of optical saccharimeters.
The polariscope of Mitscherlich was one of the earliest in use. It has
now been entirely superseded by more modern and accurate instruments.
=69. The Laurent Instrument.=—A polariscope adapted by Laurent to
the use of monochromatic yellow light is almost exclusively used in
France and to a considerable extent in this country. In case a worker
is confined to the use of a single instrument, the one just mentioned
is to be recommended as the best suited to general work. It has the
second nicol, called the analyzer, movable and the degree of rotation
produced is secured in angular terms directly on a divided circle. The
scale is graduated both in angular measurements and in per cents of
sugar for a definite degree of concentration of the solution and length
of observation tube. The normal solution in the laurent instrument
contains 16.19 grams of pure sugar in 100 true cubic centimeters, and
the length of the observation tube is 200 millimeters. Both the angular
rotation and the direct percentage of sugar can be read at the same
time. Great accuracy can be secured by making the readings in each
of the four quadrants. The light is rendered yellow monochromatic by
bringing into the flames of a double bunsen, spoons made of platinum
wire, which carry fragments of fused sodium chlorid.
[Illustration: FIGURE 33. LAURENT LAMP.]
=70. The Laurent Burner.=—The theory of the illumination of the laurent
burner is illustrated by the accompanying Fig. 33. The lamp consists
essentially of two bunsens, surmounted by a chimney.[38] A curved
spoon made of platinum gauze serves to hold the fused particles of
sodium chlorid which are used to produce the yellow light. The spoon
is shown at G, held by the arm F, fastened by the key P. The interior
intense flame B B is surrounded by an exterior less highly colored
flame A A. It is important that the optical axis of the polariscope
be directed accurately upon the disk B, which is the most intense
part of the illumination. The point of the spoon carrying the salt
should be coincident with the prolongation of the lamp TT, so that it
just strikes the edge of the blue flame. Care should be taken not to
press the spoons into the interior of the flame as by so doing the
intensity of the illumination is very much diminished. Great care must
be observed in the position of the spoon G, and the platinum arm F
being flexible, the operator with a little patience, will be enabled
to properly place the spoon by bending it. Moreover, if the spoon be
pressed too far into the flame, the melted particles of salt collecting
in the bottom of G may drop into the lamp and occlude the orifices
through which the gas enters. The light of the yellow flame produced by
the lamp may be further purified by passing through a prism filled with
a solution of potassium dichromate, or better, a homogeneous disk cut
from a crystal of that salt.
Since the flame produced by the above device is not perfectly constant,
being more intense at the moment of introducing a fresh portion of
the fused salt, the author has used a lamp designed to furnish an
absolutely constant flame.[39] This device which is shown in Fig. 34,
is based on the principle of adding constantly a fresh portion of the
salt to the flame. The flame is thus kept perfectly uniform in its
intensity.
The lamp consists essentially of two wheels with platinum gauze
perimeters and platinum wire spokes, driven by a clock-work D, and
mounted by the supports AAʹ as shown in the figure. The sodium salt,
chlorid or bromid, in dilute solution, is placed in the porcelain
crucibles F, supported by BBʹ as indicated in the figure, to such a
depth that the rims of the platinum wheels dip beneath the surface as
they revolve. The salt is volatilized by the lamp E. By means of the
crossed bands the wheels are made to revolve in opposite directions
as indicated by the arrows. The solution of the salt which is taken
up by the platinum net-work of the rim of the wheel, thus has time
to become perfectly dry before it enters the flame and the sputtering
which a moist salt would produce is avoided. At every instant, by this
arrangement, a minute fresh portion of salt is introduced into the
flame with the result of making a perfectly uniform light which can
be used for hours without any perceptible variation. The mechanism of
the apparatus is so simple that no further description is necessary.
The polariscope should be so directed toward the flame as to bring
into the field of vision its most luminous part. The platinum wheels
are adjustable and should be so arranged as to produce between them an
unbroken yellow flame. The wheels are eight centimeters in diameter and
are driven at a rate to make one revolution in from six to ten minutes.
[Illustration: FIGURE 34. LAMP FOR PRODUCING CONSTANT MONOCHROMATIC
FLAME.]
=71. Construction of Laurent’s Apparatus.=—The shadow polariscope
invented by Laurent is constructed as follows: The polarizer is a
special nicol which is not fixed in its position, but is so arranged
as to be turned through a small arc about its axis. By this device,
the quantity of light passing through it can be regulated, and the
apparatus is thus useful with colored solutions which are not easily
cleared by any of the common bleaching agents. The greater the quantity
of light admitted, however, the less delicate is the reading of the
shadow produced. The plane of polarized light emergent from this prism,
falls on a disk of glass half covered by a thin lamina of quartz which
thus divides the field of vision into halves. It is this semi-disk of
quartz which is the distinguishing feature of the apparatus.[40] The
polarized light thus passes without hindrance the half field of vision
which is covered by the glass only, but can not pass the quartz plate
unless its axis is set in a certain way. The field of vision may be
thus half dark, or both halves may be equally illuminated or equally
dark according to the position of the nicol analyzer which is freely
movable about its axis and carries a vernier and reading glass over
a graduated circle. The field of vision in the laurent may have any
of the following forms.[41] Let the polarizer be first so adjusted
that the plane of polarization of the transmitted pencil of light is
parallel to the axis of the plate lying in the direction A B. The two
halves of the field of vision will then appear equally illuminated in
every position of the analyzer. But if the polarizing nicol be inclined
to AB at an angle a, the plane of polarization of the rays passing
through the quartz plate will undergo deviation through an equal angle
in the opposite direction.
[Illustration: FIGURE 35. FIELD OF VISION OF A LAURENT POLARISCOPE.]
It happens from this, that when in the uncovered half of the field, the
plane of polarization has the direction AC, in the other half it will
have the direction ACʹ. When the analyzer is rotated, if its plane of
polarization lie in the direction cc, the rays polarized parallel to AC
will be completely extinguished and the corresponding half of the field
will be dark. The opposite happens when the plane of polarization lies
in the direction of cʹcʹ. When one-half of the field is thus obscured,
the other suffers only a partial diminution in the intensity of its
illumination. When the middle position bb is reached in the rotation of
the analyzer, the illumination of the two halves is uniform, and this
is the point at which the zero of the scale is reached. The slightest
rotation of the analyzer to the right or left of this neutral point
will cause a shadow to appear on one of the halves of the field, which
by an oscillatory movement of the analyzer, seems to leap from side
to side. The smaller the angle _a_ or BAC, the more delicate will be
the shading and the more accurate the observation. This angle being
adjustable by the mechanism already described, should be made as small
as will permit the admission of the quantity of light requisite for
accurate observation.
The various pieces composing the polariscope are arranged in the
following positions, beginning on the right of Fig. 36, and passing to
the left, where the observer is seated.[42]