Improvement of Polarimetric Apparatus in the Mid-19th Century
A polarimeter needs a single beam of plane polarized light. A reflective surface provides a single beam whose intensity is much below that of the incident light and is generally insufficient for practical measurements. On the other hand, two equally intense plane polarized beams -- one too many -- are obtained by passing natural light through a doubly refracting crystal, and one needs to be eliminated. The Nicol prism, which is made entirely of calcite, a doubly refracting mineral, isolates one beam cleanly. The prism consists of two pieces from a diagonally sliced rhombohedron of the mineral, that are cemented together (Fig. 3). Only one beam emerges from the prism. The other is reflected aside by the cement layer as a consequence of the refractive indexes of the two polarized beams which differ in calcite and of the cement which is intermediate. Polarimeters employing two Nicol prisms appeared in the 1840s (Fig. 4). The prism near the light source serves as the "polarizer." The other, the "analyzer," is mounted on a rotatable plate that is lined with a circular scale to enable the direction and degree of rotation to be observed.
A measurement with a polarimeter thus equipped begins with the prisms at the zero point, i.e., at the position of minimum observable light. Next a tube fitted with glass end-plates and filled, for example, with a solution made with measured quantities of sugar and water, is installed in the compartment between the prisms where light from the polarizer can pass through the length of the tube. The analyzer is then rotated to the light-extinguishing position at which point the rotation is recorded. For substances in solution, Biot's quantitative relationship is
where is the molecular rotatory power of the pure substance, is the observed rotation of the solution in circular degrees, c is the grams of substance per 100 mL of solution, and l is the length of the solution in decimeters. Since c and l are known constant, the value found for indicates the purity of the sugar sample tested, impure samples giving proportionately smaller values than pure sucrose. Sample purities were read directly from the observed rotations when, by convention later in the century, rotations were measured on the International Sugar Scale, where 100º S refers to the optical rotation of a beam of polarized light measured on passing through a 200 mm tube filled with a solution prepared with 26 g of pure sucrose dissolved in water to a volume of 100 mL at 20 °C.
Optically active substances like sucrose exhibit optical rotatory dispersion; i.e., their optical activity differs at different wavelengths. Hence, polarized light from a monochromatic source, a troublesome requirement in the 19th century, was needed to standardize measurements of sucrose made with a polarimeter. The development in 1845 of the saccharimeter, a polarimeter to which an adjustable double-quartz wedge added, obviated the need for a monochromatic source (Figure 5). This development exploited the fact that, except for their opposite direction of rotation, the optical rotatory dispersions of quartz and sucrose are nearly identical. Thus an appropriate thickness of quartz, as provided by adjusting the double-quartz wedge, could compensate for the rotations of sucrose at all wavelengths, making polarized light from a white-light source appropriate for sucrose measurements.
Later in the 19th century, the half-shade principle was incorporated into polarimetric instruments. The required division of the visual field into hemispheres was achieved by modifying the analyzer prism in such fashion that a fixed so-called "halfshade" angle would exist between the planes of polarization of the light in each hemisphere. A measurement would be recorded at the position where the observer located a uniformly intense light minimum in both hemispheres in place of adjacent light and dark hemispheres. Because this type of end-point is easier to locate and reproduce, the precision of polarimetric measurements improved; but, as the Customs Service's measurements showed, the precision attainable remained far from optimum. The problem was the halfshade angle, which strongly affects the intensity of the light observable by the viewer, the size of which was fixed by the maker of the saccharimeter. The fixed angle size was optimum for the appropriate illumination of neither dark solutions, which need wider angles to allow sufficient light through for the measurement, nor for clear samples, which need smaller angles to reduce the light to an appropriate intensity. Bates recognized the fixed halfshade angle, imposing a lack of adjustable sensitivity, as the major problem limiting the measurement capability of saccharimetry. In order to provide adjustable sensitivity, the size of the halfshade angle would have to be under the control of the analyst and when changed should have no effect on the zero point.
Figure 3 - Nicol's prism, by which one of the polarized rays is eliminated from the field.
Bates, F. J., et al., Polarimetry, Saccharimetry,
Figure 4 - Biot's apparatus improved by using nicol prisms as polarizer and analyzer, and using various arrangements of lenses.
Bates, F. J., et al., Polarimetry, Saccharimetry, and the Sugars, Nat. Bur. Stand. (U.S.), Circ. 440, (1942).
Figure 5 - Addition of the double quartz-wedge compensator, enabled white light to be used and laid the foundation for the modern saccharimeter. S represents the light source; N, nicol prisms; T, the tube containing the sugar solution; L1 and L2are lenses; E the observer's eye; and W, the quartz-wedge.
Bates, F. J., et al., Polarimetry, Saccharimetry,
and the Sugars,
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