264 
DE. T. M. LOWRY: NATURAL AND MAGNETIC ROTATORY DISPERSION 
The methods used in measuring rotatory dispersion differ from one another mainly in two respects:— 
1. As regards the source of light, which may give (a) a continuous spectrum or (b) a line-spectrimi, 
either monochromatic or multichromatic j 
2. As regards the methods used in selecting a line or portion of the spectrum, which may depend 
(a) upon a spectroscope whereby the light is resolved either before or after it passes through the 
polarimeter or (b) upon the use of absorption-screens. 
Broch’s Method .—The earliest measurements of rotatory dispersion were made on quartz with the help 
of sunlight falling upon a Amrtical slit; after passing through a simple polarimeter (consisting of movable 
polariser, quartz plate and analyser) the light was resolved by means of a single glass prism, without a 
telescope or eye-piece; the polariser was rotated until a dark extinction-band was superposed on one after 
another of the Fraunhofer lines from B to G (Broch, ‘Ann. Chim. Phys.,’ 1852, vol. 34, 119-121 ; from 
‘ Repertorium der Physik,’ 1846, vol. 7, 113). The same method was suggested in general terms a year 
previously by Fizeau and Foucault (‘Comptes Rendus,’ 1845, vol. 21, 1157). This method was 
improved by G. Wiedemann (‘ Pogg. Ann.,’ 1851, vol. 82, 215-232), who added a telescope to the 
refracting prism, and with this apparatus made the first exact observations of the natural rotatory 
dispersion of licpiids (turpentine and lemon oil for the Fraunhofer lines B to G) and of their magnetic 
rotatory dispersion (carbon disulphide and turpentine for the Fraunhofer lines C to G), and discovered 
the proportionality of the two rotatory powers over a series- of wave-lengths (turpentine for the Fraunhofer 
lines C, D, E, B, F gave the ratios 4'07, 4’12, 4'10, 4’14, 4‘12). A. Arndtsen used a cross-wire in the 
telescope to locate the positions of the Fraunhofer lines and measured the rotatory dispersion of sugar, 
tartaric acid,* malic acid and camphor. J. Stefan (‘Wien. Ber.,’ 1864 [IL], vol. 50, 88-124) developed 
this method of worldng into its final form by using, in series with a simple polarimeter, a complete 
spectroscope, with a prism of crown-glass, flint-glass, water or quartz, or a grating, to refract the light; he 
used an artificial source of light, t measured the positions of the extinction-bands on the circle of the 
goniometer (polariser and analyser fixed) and, after establishing a linear relationship between the rotatory 
power of the quartz and the refraction of the prism, calculated the rotatory power of the quartz for the 
seven Fraunhofer lines B to H. Stefan showed that, whilst the rotatory dispersion could not be 
represented by Biot’s formula a = a satisfactory concordance between theory and experiment could 
be obtained by using' Cauchy’s formula a = Aq -f AqA"-. Stefan’s method! was used by Y. von Lang 
(‘Wien. Ber.,’ 1875 [IL], vol. 71, 707-714) in measuring the influence of temperature upon the optical 
rotatory power of quartz ; he calibrated the spectroscope by means of lithium, sodium and thallium flames 
and brought the extinction-bands to these three positions by rotating the analysing Nicol prism. A 
complete spectroscope in series with a simple polarimeter was also used by Soret and Sarasin (‘Geneva 
Archives,’ 1882 [HL], vol. 8, 5-59, 98-132, 201-228) with solar light as illuminant and Fraunhofer lines 
as standards of wave-length in their measurements of optical rotation in quartz; the same arrangement 
was used by Guye (‘Geneva Archives,’ 1889 [HI.], vol. 22, 130) vvdth artificial light in his measurements 
of optical rotation in sodium chlorate. 
Lirricu’s 2Iethod .—In order to read rotations which are too small in magnitude to give rise to narrow 
bands, Lippich (‘ Wien. Ber.,’ 1885 [IL], vol. 91, 1070) passed the light through a spectroscope before it 
reached the polarimeter; the spectrum, instead of being received by an eye-piece, fell upon a second slit. 
* Biot, who discovered the phenomenon of anomalous rotatory dispersion in tartaric acid (‘ Mem. 
Acad. Sci.,’ 1838, vol. 15, 93; vol. 16, 229; ‘Ann. Chim. Phys.,’ 1844, (3), vol. 10, 5, 175, 307, 385; 
vol. 11, 82; 1850, vol. 28, 215, 351 ; vol. 29, 35, 341, 430), merely used blocks of the spectrum, in the red, 
yellow, green, &c., without attempting to locate accurately their positions in the spectrum. 
t Landolt (‘ Optical Rotating Power,’ 1902, p. 423) attributes to A"on Lang the method described by 
Stefan eleven years previously of using artificial light in place of sunlight. 
