26‘2 
DE. T. M. LOWEY: NATUEAL AND ^lAGNETIC EOTATOEY DISPEESION 
wave-lengths, extending over the whole of the available range of the spectrum, and 
that the measurements shall be made with the highest degree of accuracy. 
The present investigation of rotatory dispersion, which has been in progress con¬ 
tinuously since 1905, is concerned with the natural rotatory power of crystals and of 
optically active liquids, and also with the rotatory power induced in them by a 
magnetic field. It was undertaken primarily from the chemical standpoint with a 
view to obtaining information as to the variations of rotatory power with wave¬ 
length in organic liquids. But it was soon found that the two lines of investigation 
indicated in the preceding paragraph were inseparable, since the only satisfactory 
way of establishing standard laboratory methods for everyday use was to make 
measurements of rotatory dispersion with light of every available wave-length and 
to strive persistently to increase the degree of accuracy attained. Only in this way 
could adequate experience be gained as to what was possible in work of this kind. 
Thus, after making observations with light of some thirt}^ wave-lengths, it was 
found that measurements of the highest degree of accuracy could be made with 
twenty-four lines in the spectra of the elements Li, Na, Tl, Zn, Cd, Hg, Cu, Ag ; 
nine of these were used in the earliest series of laboratory-measurements, but the 
number was soon reduced to seven and finally to four (or even two) lines, which were 
found to give an adequate representation of the rotatory dispersion of the simpler 
organic compounds. 
The measurements which are now described of the rotatory power -of quartz for 
twenty-four wave-lengths in the visible region of the spectrum were undertaken 
originally in order to test the methods which were being devised for general laboratory 
use. But it was soon apparent that measurements might be made which would be 
much more accurate than those that had been recorded hitherto, even including the 
measurements of Soret and Sarasin which have been accepted as standards during 
the past thirty years. The observations were therefore continued and extended in 
order to provide material which could be used in testing with the greatest stringency 
the relationships between rotatory power and wave-length which have been put 
forward by Drijde and others. Measurements of magnetic rotatory dispersion in 
quartz were also made, in order to test, under more stringent conditions than those 
which have generally been adopted, the validity of G. Wiedemann’s Law of the 
proportionality of natural and magnetic rotatory dispersions. 
Experiments are in progress to extend the measurements of the natural rotatory 
power of quartz in such a way as to cover the whole of the spectrum transmitted by 
the mineral. Up to the present, measurements have been made from X 17000 to 
A 2327 ; it is hoped that, by the adoption of new methods, it may be possible 
ultimately to carry the experiments into the unexplored regions lying beyond these 
limits, but this final extension will be accomplished only after overcoming a number 
of new and serious difficulties. 
Measurements have also been made of natural and magnetic rotatory dispersion 
