stance per 100 g of solution, and c is the grams per 100 ml of solution. It is the hypothetical rotation produced in unit length by 1 g of the active material dispersed in a volume of 1 ml, which in present day nomenclature, is termed "specific rotation." Molecular rotation is the hypothetical rotation produced in unit length by 1 gram-molecule of the active material dispersed in (or condensed into) a volume of 1 ml.

This work of Biot appears to have been the foundation of all polarimetry. From this time on, rapid strides were made in the improvement of apparatus, and there resulted the science of Polarimetry as we know it today. A few years later, the polarizing plate of black

FIGURE 4.-Original form of the nicol prism.

A, Diagrammatic sketch; B, detailed sketch of limiting rays (1 and 2) and of the various angles. DAC and BCA are right angles.

glass and the achromatized calcite analyzer rhomb had been replaced by the nicol [10] prism, previously mentioned, which was a far more effective and convenient device for polarizing light than the glass plate. It consisted of a rhomb of Iceland spar cut diagnonally and the two pieces cemented together in such a manner that one of the doubly refracted rays is reflected to one side, and the other passes on through the prism.

(b) TYPES OF POLARIZERS (FIG. 6) AND POLARISCOPES (FIG. 7)

The simple polariscope of Biot was improved by Mitscherlich [11] and by Ventzke [13], one of whom appears to have been the first to use two nicols, and by Robiquet [12] who added the Biquartz of Soleil [14], thus making the setting dependent upon the transition tint (tiente de passage), and many others. Some transition tint instruments are still in use.

In 1856 Pohl [15] attempted to increase the sensitivity of the simple polariscope by the use of a mica halfshade, but was not entirely

successful. However, his idea was later satisfactorily developed by Laurent [16]. In 1845 Soleil [14] had invented the quartz-wedge compensating system and added it to the polariscope, thus laying the basis for the modern saccharimeter.

In 1860 Reverend William Jellet [17] described the first satisfactory halfshade polariscope. The utilization of the halfshade principle was the first important step in the perfection of the modern instrument. Prior to this, with the exception of the Soleil biquartz transition tint plate, both the polarizing and analyzing devices were simple nicol prisms, the "end point" being determined by setting the instrument for a minimum of light intensity in the field. Jellet's contribution introduced into polariscope design the photometric field. To construct his halfshade device, which he used as the analyzer, he selected a rhomb of Iceland spar several times longer than its other dimensions, and squared off the end faces. He then sliced the resulting prism parallel to its long dimension, BS' and at a small angle, SCD, to the short diagonal D'D of the end faces (as indicated in fig. 6A), reversed the two pieces end for end, and cemented them together (fig. 6B). In this manner he obtained a compound prism (shown in cross section in fig. 6C) whose principal crystallographic section A'C in one half made a small angle to that (ČA) in the other half, the principal crystallographic section of each half being equally inclined to the short diagonal of the end face. Diaphragms were placed centrally at each end of the prism. One ray (the ordinary ray O' and 0) came straight through, while the other (the extraordinary E' and E) was deviated slightly in each half and diaphragmed out. Thus the light in one half of the field was polarized in a plane making a small angle (about 2°) with the plane of polarization of the light in the other half of the field.

In 1870 Cornu [18] improved upon Jellet's idea by removing a wedgeshaped section from a nicol prism and recementing the two halves. Thus the plane of polarization in each half of the field made a small angle with that in the other half, as in Jellet's prism.

Schmidt and Haensch [19] simplified Cornu's prism by removing the wedge-shaped section from one half only of the nicol prism, the three pieces then being cemented together as before. In mounting, the divided half was placed toward the analyzer, and each part of this half gave light vibrating in a plane, which made an angle equal to the angle of the removed section, with the plane of vibration of the light coming from the other part. This angle is known as the halfshade angle. The field of the instrument thus appears divided into two parts, and the setting is made by turning the analyzing nicol until the parts become of equal intensity. The sensitivity of the instrument depends on the magnitude of the angle of the halfshade. As the angle diminishes, the precision with which a setting can be made. increases. However, the total illumination of the field diminishes with the halfshade angle. An angle of about 2.5° is the minimum working value for ordinary conditions of measurement.

The advantage of the halfshade principle was universally recognized, and a number of halfshade polarizing systems were introduced by various investigators. Three of these, the Laurent, the Lippich, and the Jellet-Cornu as modified by Schmidt & Haensch, have been extensively used by polariscope builders.

FIGURE 5.-Graphic representation of steps in the early development of saccharimetry.

A, (1669) The simple calcite rhomb (Bartholinus).

B, (1783) Rochon's double-image prism.

C, (1808) Malus, polarization by reflection.

As a

D, (1810-11) Arago's discovery of the rotation of the plane of polarization. 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