IN QUARTZ OF LIGHT IN THE VISIBLE REGION OF THE SPECTRUM. 
263 
in some fifty or sixty paraffinoid compounds, including a series of twenty-three 
optically-active alcohols prepared by Dr. E;. H. Pickard ; these will be described in 
a separate communication as soon as two series of active alcohols now under examina¬ 
tion have been completed and tested. 
2. General Review of Previous Observations and Methods. 
Although much work has been done on the optical rotatory power of organic compounds, it is remark¬ 
able that almost the whole of the observations have been made with light of one colour—the yellow 
doublet of sodium. Observations have been made occasionally with light of other w’avedengths, usually 
by physicists who have proceeded from the study of quartz or of sodium chlorate to analogous observations 
on the rotatory dispersion of some optically active liquid such as turpentine or ethylic tartrate. But 
the measurement of rotatory dispersion has never become a part of the normal work of the chemical 
laboratorju Chemists have been content to devote their energies to the elucidation of the effect of 
solvents, of temperature and of chemical constitution on the rotatory power of substances for sodium light, 
usually without paying attention to the influence of the nature of the light on the property in question.* 
This limitation is the more remarkable since many of the substances selected for examination are knowm 
to exhibit anom.aloiis rotatory dispersion, and cannot, therefore, be expected to exhibit any simple relation¬ 
ships until attention is directed to the w'hole course of the rotatory dispersion-curve rather than to any one 
arbitrarily selected point. 
Measurements of magnetic rotatory dispersion have been even more uncommon. Perkin made a few 
scattered observations with lithium and thallium in addition to sodium light; a few measurements have 
also been made in order to determine the form of the dispersion-curve or to test the validity of 
Wiedemann’s Law' of the proportionality between natural and magnetic rotatory dispersion, but 
practically the whole of the literature of magnetic rotatory power, like that on natural optical rotation, is 
expressed in terms of the effects produced on yellow sodium light. 
The influence of wave-length on the optical rotatory power of crystals has been studied much more fully 
than in the case of liquids. Of some thirty substances which show this property, about one-third have 
been examined wdth light of three or more wave-lengths. In the case of quartz and of sodium chlorate, 
the observations have been extended by Soret, Sarasin, and Guye (‘Geneva Archives,’ 1882 [HI.], 
vol. 8, 5-59, 98-132, 201-228; 1889 [HI.] vol. 22, 130) from the visible to the ultra-violet region of the 
spectrum. Observations of the optical rotatory power of quartz in the infra-red ‘region have been made 
by E. Carvallo (‘Comptes Rendus,’ 1892, vol. 114, 288) and by R. Dongier (‘Comptes Rendus,’ 1897, 
vol. 125, 228). As questions of solvent, concentration and chemical constitution enter scarcely at all into 
the study of the optical rotatory power of crystals, this property has been investigated mainly from the 
standpoint of crystallography or of pure physics. In addition to the influence of the wave-length of the 
light, the effect of temperature has been the problem mainly studied. In particular, the optical rotation 
produced by quartz plates has been measured over the range of temperature from -[-840° C. (JOUBERT, 
‘Comptes Rendus,’ 1878, vol. 87, 497) to - 190° C. (Molby and Gibbs, ‘Rhys. Rev.,’ 1910, vol. 30, 77-91; 
vol. 31, 291-310). 
* See, for instance. Prof. Frankland’s review of the subject in his Presidential Address to the 
Chemical Society, March 28, 1912. 
Note.- —It does not appear to be generally known that chemists and physicists have adopted different 
conventions as regards the sign of an optical rotation, so that dextro-quartz and dextro-camphor actually 
rotate the plane of polarisation in opposite directions. In the case of dextro-quartz the rotation is clock- 
wdse to an observer looking along the path of the light; in the case of dextro-camphor the rotation is 
clock-wise as viewed from the eye-piece of the polarimeter. 
