A, (1669) The simple calcite rhomb (Bartholinus). B, (1783) Rochon's double-image prism. C, (1808) Malus, polarization by reflection. D, (1810-11) Arago's discovery of the rotation of the plane of polarization. As a polarizer, he used a plate of glass, and as an analyzer, a calcite prism, achromatized with a piece of glass, operating upon the principle of Rochon's doubleimage prism with which Arago was familiar. E, (1815-17) Biot's discovery of the proportionality of the rotation to thickness and concentration, and the formulation of the physical laws upon which modern polarimetry is based. He used apparatus similar to Arago's, placing his solutions in a metal tube closed with glass end plates or cover glasses. G represents the glass plate; T, the tube; and R, the simplified Rochon prism. F, (1827) Nicol's prism, by which one of the polarized rays (as in the Rochon prism) is eliminated from the field. G, (1842) Mitscherlich, also Ventzke, improved upon Biot's apparatus by using nicol prisms as polarizer and analyzer, and using various arrangements of lenses, and applied it in the sugar industry. H, (1845) Soleil's biquartz or sensitive tint plate was added, giving a divided field in which the setting of the instrument was made by matching the colors of the two halves of the field at the "sensitive tint" point. I, (1845) Soleil's double quartz-wedge compensator was added, enabling white light to be used and laying the foundation for the modern saccharimeter. In G, H, and I, S represents the light source; N, nicol prisms; T, the tube containing the sugar solution; L1 and L2, lenses; E, the observer's eye; P, a quartz plate; Q, Soleil's biquartz; and W, Soleil's quartz-wedge compensator. When plane polarized light of more than one wave length traverses an optically active substance, it emerges with the vibration planes of the individual waves all inclined to each other in a sort of fanshaped arrangement, the violet waves having been rotated through the greater angle. An analyzing nicol, on being rotated, will cut out FIGURE 6.-A, B, and C, Jellet's halfshade device which made the setting dependent upon matching intensities of the light in the halves of the polariscope field; D, Lippich-type halfshade device. each wave in turn, but it is impossible to darken the field owing to the fact that some portion of the light from each of the remaining waves passes through the analyzer. If the polarizer be of the divided-field type, it will be found impossible to darken either half of the field, and color will always be present. Thus, in all polariscopes in which the setting is made by rotating the analyzing nicol, it is necessary that the light source be monochromatic, or at least nearly so. (1) LAURENT AND JELLET POLARIZERS [16,17]. Of the three halfshade polarizing systems mentioned above, the Laurent alone is limited to the use of one single monochromatic source. In this system a half-wave plate, usually a thin plate of quartz cut parallel to the optic axis, covers one-half of the field of the polarizing nicol. In order that the two rays traversing the doubly refracting quartz can combine to give plane polarized light on emergence, they must have an optical difference of path equal to one-half wave length. Thus, the thickness of the quartz must always be such as to bring this condition about, and this form of halfshade can be used only with a light source giving the particular wave length for which this condition is fulfilled. The angular position of the new plane of polarization is slightly different from that of the polarizing nicol, and the conditions for a halfshade, are thus established. The advantage of this system is due to its adjustable sensitivity; the halfshade angle, being twice the angle between the optic axis of the plate and the plane perpendicular to the principal section of the polarizer, can be readily varied by rotating the polarizer. Inasmuch as the Laurent polarizer requires a monochromatic source, it is seldom combined with a quartz compensating system. The Jellet polarizer, described above, has a fixed halfshade angle, and therefore the sensitivity cannot be varied. Its advantage lies in the fact that it does not easily get out of adjustment, and does not, in itself, require the use of monochromatic light. (2) LIPPICH SYSTEM [20].-In the Lippich polarizing system, the halfshade angle is formed by two beams of plane polarized light which come from two separate nicols, one of which covers but one-half of the aperture of the other (fig. 6D). If these two nicols are turned until the vibration planes of the light which they transmit coincide, they act as a single nicol (with some reservations). If one of them be rotated through any angle, a halfshade angle is formed equal to that angle. Because of the ease with which the halfshade angle can be varied, as well as the high degree of perfection attained by the opticians in constructing the prisms, we have in the Lippich an adaptable and sensitive polarizing system. The accuracy with which a setting can be made is increased to the extent that the dark dividing line between the halves of the field can be made to vanish. With a broad source of light this condition is very nearly attained in the Lippich. It does not in itself require the use of a monochromatic source. (3) SENSITIVE-STRIP SYSTEM.-In 1903 Brace [21] described the sensitive-strip spectropolariscope. In the ordinary nicol prisms the extraordinary ray is utilized. It occurred to Brace that it was possible to reverse this condition and use the ordinary ray. Thus, instead of a film of liquid between two large pieces of Iceland spar, he proposed using a thin piece of spar immersed in liquid and placed in a cell with glass ends, the plate of Iceland spar covering the entire field and being inclined at an angle of 70° to the axis of the system. To obtain a polarizing system similar to the Lippich, it would be necessary to place a second cell with a narrow strip of spar covering one-half of the field, in the position ordinarily occupied by the small nicol of a Lippich system. If such a system could be perfected, it would have many advantages over the Lippich. Among these may be mentioned |