SECTION III., 1911. [13] Trans. R. S. C. 
On the Rotatory Dispersion of Quartz. 
By Proressor H. F. Dawss, M.A. 
Presented by Prof. J. C. McLennan. 
(Read May 16, 1911.) 
The following experiment constitutes a method of determining 
the Rotatory Dispersion of a substance, such as quartz, which has a 
relatively large rotatory power. In it light is detected by photographic 
means and the method is, therefore, applicable not only to wave lengths 
of the visible spectrum but also to the parts of the ultraviolet to which 
one may have access. An essential part of the arrangement of the appara- 
tus consists of a spectrograph, so that the experiment may seem to il- 
lustrate the relative efficiencies of different types of apparatus in investi- 
gations ranging over a large part of the light spectrum. 
The method depends on the use of a plate of the optically active 
substance which is sufficiently thick to rotate the plane of polarisation 
many times 180°. The rotation varies with the wave length and 
increased as the wave length decreases. If, therefore, for a certain 
wave length Xo (in the extreme red, say,) there is a rotation A,, there 
will also be rotations A, + 180, A, + 2 x 180....A, + n. 180 for 
wave lengths À,,À,,....A, which are less than A, and in decreasing order 
of magnitude. Hence, if the plate of substance is placed between 
analyser and polariser and if the latter is set to extinguish the wave 
length y, it will also extinguish the wave lengths À,, A,....,, and in 
the spectrum of the analysed light there will be darkness at the positions 
of these colours. | 
In order to exhibit the relation between the rotation and the wave 
length, therefore, it will be necessary to determine the values of A,, and 
of the various wave lengths coresponding to the places of darkness in 
the spectrum. The value of A,, which is equal to a+} 180b (b a whole 
number, a less than 180°) may be obtained by a direct saccharimetric 
measurement; a is given by the setting of the analyser and b may be 
inferred from the thickness of the plate by comparison with plates of 
small thickness. The values of the several wave lengths are given at 
once by the calibration curve of the spectroscopic system. 
EXPERIMENTAL. 
The rotation of the plane of polarisation was found for quartz 
using a specimen of thickness 3-85 cms. The sources of light used 
