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 |