296 
DR. T. M. LOWRY: NATURAL AND MAGNETIC ROTATORY DISPERSION 
magnetic rotatory dispersion is a little higher in the vitreous than in the crystalline 
state :— 
Rokitions. 
Ratios. 
Li . . 
. . 6708 
r*i9 
0*63 
Hg . . 
. . 5461 
r*89 
1*00 
Hg . . 
. . 4359 
3°*17 
1*67 
An equal length of quartz gave a rotation 1°’98 instead of 1°’89 for wave¬ 
length 5461. Later attempts to secure a rod of clear silica 100 mm. in length for 
accurate measurements of the magnetic rotatory dispersion, were not successful; 
although the material contained fewer bubbles than the discs prepared a few years 
before, the triple field of the polarimeter could not be seen through the rod, which 
gave instead a splendid demonstration of Airy’s spirals. 
As contrasted with the exact accordance, throughout the range from 6708 to 4359, 
of the two series of dispersion-ratios for quartz, it may be noted that the nearest 
approach to equality in a series of 23 alcohols was found in the alcohol phenylmethyl- 
carbinol which gave for the ratio 4359/5461 
Magnetic 1739, Optical 1736. 
But the accidental nature of this concordance is clearly shown by the fact that the 
next homologue phenylethyl-carbinol gave the values 
Magnetic 1731, Optical 1’674. 
10. Summary. 
(1) A large crystal of optically perfect Isevo-quartz has been found which produced 
a rotation of 25°'537l per mm. in light of wave-length 5460*97. Cylinders, also 
apparently perfect, cut from a crystal of dextro-quartz, produced a rotation of 
25°*5361 per mm. Cylinders cut from plates of dextro- and of Isevo-quartz which 
gave the figure 25°*5338 per mm. were shown to be faulty by a new test, in which 
the specimen, mounted between Nicol prisms set to extinction, is examined with 
monochromatic green mercury light, with the help of a telescope focussed on the 
interior of the crystal. A very sensitive test of the optical purity of quartz consists 
in measuring its optical rotatory power for mercury green light with both a positive 
and a negative half-shadow angle. 
(2) The rotatory dispersive power of quartz has been determined by measuring the 
rotation produced in light of 24 wave-lengths from Li 6708 to Hg 4359, using a 
column of Isevo-quartz 226*3670 mm. in length and a column of dextro-quartz 
181*4382 mm. in length. The average error of the readings amounts to about 6 parts 
per million; the average error from all sources in the absolute values is probably not 
more than 0°*001 per mm. 
