4. Measurement of the axis error. If a plate satisfactorily passes these tests, its optical rotation is measured, at 20° C, using spectrally purified light of wave length 5461 A obtained from a mercury arc. The rotation for λ=5892.5 A may be obtained by direct measurement, or by multiplying the rotation for the Hg light by 0.85085. The sugar value of the plate is then assigned, based upon either of the two rotations given above. The sugar value is discussed more fully below under "Saccharimeters." This Bureau reserves the right to reject any plate showing faults which tend to make it unreliable. (e) SPECIAL TESTS Additional data upon quartz control plates submitted for test may be had by special arrangement. Standardization for wave lengths other than λ=5461 A and λ=5892.5 A will be made, provided the order of accuracy desired is consistent with the intensity and purity of the source available. 4. REFERENCES [1] Erasmus Bartholinus, Experimenta Crystalli Islandici disdiaclustici quibus mira et insolita refractio detegitur (Amsterdam, 1670). [2] Christian Huygens, Traité de Lumière (Leyden, 1690). [3] l'Abbé Alexis Marie Rochon, Recueil de mémoires sur la mécanique et sur la physique (1783). Mém. [4] Etienne Louis Malus, Mém. Soc. d'Arcueil 2, 143, 149 (1808-1809). Savants étrangers 2, 303 (1810-1811). Théorie de la double refraction (Paris, 1810). [5] Dominique François Arago, Oeuvres complètes 10, 54. Mem. l'Institut, Class Math. Phys. 12, 115 (1811). [6] Jean Baptiste Biot, Mém. l'Institut, Class Math. Phys. 12, 135-280 (1811); 13, 1-371 (1812); 13, 1-18 (1813); 13, 18-38 (1814) [7] Jean Baptiste Biot, Bul. Sciences Soc. Philomatique, 190–195 (1815). [8] Jean Baptiste Biot, Mem. Acad. Sci. 2, 41-136. (1817.) [9] Jean Baptiste Biot, Sur la construction des appareils destinés a observer le pouvoir rotatoire des liquides, Ann. chim. phys. 74, 401-430 (1840). [10] William Nicol, Edinb. New Phil. J. 6, 83–94 (1828); 14, 372 (1831); 27, 332 (1839). [11] Eilhard Mitscherlich, Polarisation et Saccharimétrie, by D. Sidersky, 2d edition (1908), page 46. [12] Robiquet, Polarisation et Saccharimétrie, by D. Sidersky, 2d edition (1908), page 48. [13] K. Ventzke, Erdmann's J. prakt. Chem. 25, 65–84 (1842); 28, 111 (1843). [14] Henri Soileil, Compt. Rend. 20, 1805 (1845); 21, 426 (1845); 24, 973 (1847); 26, 163 (1848). [15] Joseph Johann Pohl, Wiener Bericht 22, 492 (1856). [16] H. Laurent, J. phys. 3, 183 (1874); 8, 164 (1879). [17] Rev. J. H. Jellet, Brit. Assn. Rep. 29, 13 (1860); Proc. Roy. Irish Acad. 7, 348 (1860); Proc. Roy. Irish Acad. 8, 279 (1863); Trans. Roy. Irish Acad. 25, 371-450 (1875); Ž. ver. Rubenzuckerind. 15, 456 (1865). [18] Alfred Cornu, Bul. Soc. Chim. Paris 14, 140 (1870). [19] H. Landolt, Die Polarisationsapparate Bericht über die wissenschaftlichen Instrumente auf der Berliner Gewerbeausstellung in Johre, p. 357 (1879), (Berlin, 1880). [20] F. Lippich, Z. Instrumentenk. 2, 176 (1882); 14, 326 (1894); Weiner Ber. 91, 1059 (1885). [21] D. B. Brace, Phil. Mag. 161-170 (1903). [22] F. Lippich, Z. Instrumentenk. 12, 340 (1892). [23] H. Landolt, Ber. deut. chem. Ges. 27, 2872 (1894). [24] F. J. Bates, Bul. BS 2, 239 (1906) S34. [25] H. Ebert, Wied. Ann. 34, 39 (1888). [26] H. Landolt, Optical Rotation of Organic Substances, 2d edition, p. 404. Translated by J. H. Long (Chemical Publishing Co., Easton, Pa., 1902). [27] Pribram, Z. anal. Chem. 34, 166 (1895). [28] H. Landolt, Z. Instrumentenk. 4, 390 (1884). [29] General Electric Vapor Lamp Co., Bulletin 900. [30] Osram, Berlin 17, Germany. [31] Lines in the Arc Spectrum of the Elements, compiled and published by Adam Hilger, Ltd., 98 Kings Road, London, Ñ. W. 1, England. [32] Ch. Fabry and A. Perot, Compt. Rend., p. 407 (1898). [33] R. A. Houstoun, Phil Mag. 7, 456 (1904). [34] General Electric Vapor Lamp Co. Bulletin 112; also Rev. Sci. Instr. 9, 325 (1938). [35] T. M. Lowry, Phil. Mag. 18, 320 (1909). [36] F. J. Bates, Sci. Pap. BS 16, 45 (1920) S371. [37] H. J. S. Sands, Proc. Phys. Soc., London, 26, 127 (1914). [38] E. Brodhun and O. Schönrock, Z. Instrumentenk. 22, 353 (1902). [39] E. Gumlich, Wissensch. Abhandl. Phys.-Tech. Reichsanstalt 2, 212 (1895). [40] O. Schönrock, Z. Instrumentenk. 22, 1 (1902). [41] V. Lang, Pogg. Ann. 156, 422 (1875). [42] Sohncke, Wied. Ann. 3, 516 (1878). [43] Le Chatelier, Compt. rend. p. 109 (1889). IV. MEASUREMENT OF ROTATION IN SUGAR DEGREES 1. DEVELOPMENT OF THE SACCHARIMETER (a) HISTORICAL INSTRUMENTS (FIG. 17) In the development of the saccharimeter there are two factors that have of necessity received most consideration. They are, first, sufficient illumination of the field for average work and, second, the highest obtainable sensitivity consistent with meeting necessary requirements. Because of the rotatory dispersion, or the different rotation for different wave lengths, precise measurement of rotation ordinarily requires the use of a monochromatic light source. This is especially true if the precision attainable by the utilization of the photometric principle as realized in the halfshade is to be obtained. This condition holds in polariscopes designed primarily for absolute measurements of the rotation of the plane of polarization. Unfortunately a simple monochromatic source of sufficient intensity and otherwise suitable for all polarimetric work has never been realized. In order to obviate this difficulty, Soleil, a Parisian optician [1], as early as 1845 invented the first quartz-wedge compensator and applied it to the polariscope of Robiquet, permitting the use of white light illumination and obviating the necessity of using monochromatic light. He used a double quartz wedge so arranged that it was in effect a quartz plate of variable thickness, opposite in rotation to the sugar solution being measured. Since quartz has almost the same rotatory dispersion as sucrose, this device compensated or balanced out the rotation produced by the sugar solution, wave length by wave length, and returned the vibration planes of all the different wave lengths to the original vibration direction common to all before they entered the optically active substance. Since the calibrated wedge is driven across the field until conditions are as they were before the rotating substance was placed. in the intrument, rotatory dispersion is practically eliminated and white light may be used. The absence of light sources of sufficient intensity has always been one of the most potent factors in influencing the design of saccharimeters. The higher limit of the sensitivity has been almost entirely determined by this factor. Even with the average halfshade angle of 6° to 8° the polarizing and analyzing nicols are practically crossed, so that only a mere fraction of the incident light ever reaches the eye of the observer. If the halfshade angle is decreased in order to increase the accuracy with which observations can be made, the intensity of the light transmitted is rapidly reduced. Thus monochromatic sources are inadequate in intensity when even fairly accurate settings are to be made, unless the active substance whose rotation is to be measured is quite transparent. Unfortunately this is not the case with many of the optically active liquids. This is especially true of the average raw sugar solution, and hence Soleil, as stated above, invented the quartz-wedge compensator which permits the use of white light with its relatively great intensity. (b) MODERN INSTRUMENTS (FIG. 17) The practical necessity for using white light has resulted in the quartz compensating instrument displacing practically all other types of saccharimeters, as is evidenced by the similarity in the perfected instruments of Fric; Bausch & Lomb; Schmidt & Haensch; Peters; Reichert; and others. This has been brought about despite the fact that the compensating wedge ordinarily prevents the full use of the adjustable halfshade angle of the Lippich polarizing system. Thus, while the best results in the designing of polariscopes for use with a white-light source have so far been obtained by using the Lippich polarizing system and a quartz-wedge compensation, all makes have had the great weakness of an unadjustable halfshade angle, and therefore a fixed sensitivity. Only one value of the halfshade angle can be used, and it must necessarily be large enough to give sufficient light to read, for example, the darkest-colored raw-sugar solutions. When polarizing substances having a small coefficient of light absorption, such as the better grades of sugars, are used, in which case the observer has more light than he needs, he still has available only the low sensitivity which corresponds to that value of the halfshade angle which gives sufficient light to polarize substances with a relatively large coefficient of absorption, such as very dark raw sugars. If then it were possible to retain the quartz compensation and at the same time have the halfshade angle adjustable, an advance in polariscope construction would be made comparable with the invention of the wedge. The defect due to the lack of adjustable sensitivity on a whitelight instrument has been in evidence not only in ordinary use but especially so when the saccharimeter was used for research work. The ordinary quartz compensating polariscope is utilized in practically every chemical laboratory. The highest available precision of the instrument is required in order to meet the demands of routine use. Yet the research investigator also has been compelled to depend upon it. The futility of taking a large number of observations on an instrument sensitive to 0.15 percent and using the average value as good to 0.015 percent is too well known to need discussion here. Nevertheless, the chemist has been compelled to do this because the majority of the research problems involving the use of the polariscope require the measuring of rotations with a precision greater than 0.1 percent. There can be no question that the present status of polarimetry would have been immeasurably advanced had there been a |