256 
MESSRS. THOMAS MARTIN LOWRY AND PERCY CORLETT AUSTIN 
From this time onwards attention became concentrated almost exclusively upon the 
maximum and the reversal of sign, but more especially upon the maximum, the presence 
or absence of which became almost the sole criterion for discriminating anomalous from 
normal rotatory dispersion. Thus Grossmann speaks of “ anomalous rotatory dispersion 
and change of sign of rotation ” as if they were two totally distinct phenomena ( £ Trans. 
Faraday Soc.,’ 1914, vol. 10, p. 61), whilst Winther actually limits the idea of anomalous 
rotatory dispersion to those cases in which there is a maximum in the visible region of 
the spectrum ; he therefore speaks of a dispersion-curve as becoming “ normal, in that 
the maximum passes into the ultra-violet,” whilst a curve which cuts the axis is described 
as “ normal with a maximum in the infra-red ” (’ Zeitschr. f. Physikal. Chem.,’ 1902, 
vol. 41, p. 188 ; and 1903, vol. 45, p. 337). 
A much more rational description has been given by Tschugaeff, who states that 
“ most of the colourless active bodies exhibit normal rotatory dispersion, the numerical 
value of the optical rotation continuously increasing with decreasing wave-lengths,” 
whilst the term anomalous “ is generally applied to those cases in which the optical 
rotation passes through a maximum or through a zero value or decreases with 
decreasing wave-length ” (‘ Trans. Faraday Soc.,’ 1914, vol. 10, p. 70). This 
qualitative description is in close agreement with “ An Exact Definition of Normal and 
Anomalous Rotatory Dispersion ” (Lowry, £ Trans. Chem. Soc.,’ 1915, vol. 107, 
p. 1195), which has recently been put forward as a result of exact analyses of the 
mathematical form of a large number of typical dispersion-curves. These analyses 
have shown that in practice a clear distinction may be drawn between “ anomalous ” 
curves, which cut the axis of zero-rotation, and “ normal ” curves, which do not cut the 
axis ; the normal curves rise steadily from zero towards an infinite rotation as the 
wave-length decreases, and are similar to rectangular hyperbolas in their general 
appearance ; the anomalous curves exhibit, in different regions of the spectrum, all the 
features that have been described as anomalies, including an inflexion, a maximum, a 
reversal of sign, and other related characteristics which appear simultaneously whenever 
the curve is drawn across from one side of the axis to the other. 
(c) Simple and Complex Rotatory Dispersion. 
About 1898, Drude (see ' Theory of Optics,’ 1907, p. 413), making use of the electronic 
theory of radiation, expressed the variation of rotatory power with wave-length by 
means of the general formula 
a = 1 2 = 2 }” (approximately) 
A A — A„ 
