IN QUAETZ OF LIGHT IN THE VISIBLE REGION OF THE SPECTRUM. 
295 
have been measured during the last six years. A description of the apparatus and 
methods used in these investigations will be given in a later paper dealing specifically 
with the rotatory dispersion of organic compounds. But the main results may be 
summarised in the words of a brief preliminary note, communicated to the Winnipeg 
meeting of the British Association in 1909, to the effect that “ in the case of quartz 
there is an absolute agreement between the two dispersions, but every optically active 
liquid that has been examined shows a divergence between the two series of values, 
the optical dispersion being usually but not always higher than the magnetic 
dispersion.” 
Since the publication of this note, Darmois has described (‘Ann. Chim. Phys.,’ 
1911 (VIII.), voL 22, 247-281, 495-589) a series of measurements of the optical and 
magnetic rotatory dispersion of a number of compounds of the terpene series, some of 
the measurements being carried into the ultra-violet region. He concluded that “ the 
law of proportionality was quite inexact, and that Wiedemann’s result was the 
result of a pure chance ” ; he further states that, since normal magnetic dispersion 
may accompany an anomalous optical dispersion, “ Wiedemann’s Law has no longer 
any manner of significance.” 
This conclusion, differing so entirely from that of Disch, although based upon 
observations of much the same character, is not in agreement with the conclusions 
that have been drawn from the present series of experiments. These were summed up in 
the note already referred to, in a reference to the general “ identity of the optical and 
magnetic dispersion in crystals.” The view that Wiedemann’s Law holds accurately 
for crystals but not for active liquids is at present based upon observations in the 
case of crystals of quartz only, but it is hoped at a later date to test it by analogous 
observations on sodium chlorate and other crystals. 
In measuring the magnetic rotatory dispersion of quartz 4 pieces of leevo-quartz of 
length 100‘29 mm. were set up with 4 pieces of dextro-quartz of length 98‘83 mm., 
giving a total length of 199‘12 mm. with only 1'46 mm. of laevo-quartz uncompensated. 
The following readings were obtained for the magnetic rotatory power at 20° C.;— 
Water, 200‘34 mm., gave 12°’61 for wave-length 5461. 
Quartz, 199"12 mm., gave 16°'29 ,, ,, ,, 
Dispersion - Ratios. 
Magnetic rotation. 
Magnetic. 
Optical. 
Difference. 
Li. . . 
. 6708 
10°-53 
0-646 
0-648 
+ 0-002 
Na . . 
. 5893 
13-86 
0-851 
0-851 
± 
Hg . . 
. 5461 
16-29 
1-000 
1-000 
± 
Cd . . 
. 5086 
19-01 
1-167 
1-164 
-0-003 
Cd . . 
. 4800 
20-47 
1-318 
1-319 
+ 0-001 
Hg . . 
. 4359 
26-50 
1-627 
1-627 
+ 
The foliowincr 
o 
values for 
24 mm. thickness of silica discs 
appear to indicate that the 
