ON OPTICAL ROTATORY DISPERSION. 
257 
where the “ dispersion constants ” A„ 2 , corresponding with the natural free periods 1 /a„ 
of the electrons, were deduced from measurements of refractive dispersion as expressed 
by the equation 
N 2 = E + 2 
M. 
A J -A„ 
in which N is the refractive index and E is the dielectric constant of the 
medium. 
In those cases in which only one electron need be considered, the magnetic rotation 
could be expressed by an equation 
which involved four constants, of which two could be derived from measurements of 
refraction. 
The importance of these equations did not appear immediately. Very few experi¬ 
mental data were available, and Drude applied his formulae only to the magnetic 
rotations of carbon disulphide and of creosote and to the natural rotatory power of 
quartz, which he expressed by the two-term equation 
h h 
a = —---71 
A" —Aj. A J 
containing two arbitrary constants k u k 2 , and one constant, A 1 2 , derived from measure¬ 
ments of refraction ; the dispersion constant of the second term was omitted as being 
negligible in comparison with A 2 . Perhaps on account of the lack of suitable data, the 
new formulae were not applied to any single member of the vast array of optically active 
organic compounds which have been prepared and studied, more especially from the 
time of Pasteur onwards. 
In view of the very limited application of these formulae by Drude himself, the 
indefinite number of arbitrary constants which they contained, and the fact that a 
complete knowledge of the refractive dispersion of the medium was presupposed, it is 
not surprising that Drude’s formulae remained almost barren so far as their immediate 
application to measurements of rotatory dispersion was concerned. Fifteen years of 
work on rotatory dispersion (‘ Phil. Trans.,’ A, 1912, vol. 212, p. 261 ; 1 Trans. Chem. 
Soc.,’ 1913, vol. 103, p. 1062 et seq.) have, however, provided ample data for testing 
the validity of these equations, and have established beyond question the fact that they 
are adequate to meet all the requirements of the most diverse and of the most exact 
measurements of rotatory dispersion. In the case of some scores of organic compounds, 
2 o 
VOL. CCXXII,—-A. 
