Since many persons are more or less affected with color-blindness, the shadow are to be preferred to the tint fields of vision.
For practical use in sugar analysis the white light is much more convenient than the monochromatic light.
For purposes of general investigation the polarimeters built on the model of the laurent are to be preferred to all others. Such instruments are not only provided with a scale which shows the percentage of sucrose in a solution, but also with a scale and vernier by means of which the angular rotation which the plane of vibration has suffered, can be accurately measured in more than one-quarter of the circle.
DESCRIPTION OF POLARIZING INSTRUMENTS.
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 It has now been entirely superseded by more modern and accurate instruments.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.
Figure 33. Laurent Lamp.