282 DK. T. M. LOWRY: NATURAL AND MAGNETIC ROTATORY DISPERSION 
neighbourhood of material that was obviously faulty, it was felt that some uncertainty 
might still attach to the absolute value of the observations made with it. Great 
importance, therefore, was attached to the discovery in Messrs. Hilger’s stock of a 
large block of Isevo-quartz in the form of a plate 58 mm. in thickness ; this was 
badly cracked in places but showed no trace whatever of any optical flaw in a mass 
which must have weighed at least 6 pounds. The block was so large that the 
central part was reserved for a 30° Cornu prism 5 inches in height, only the outer 
portions being used in the present investigation. These outer portions were submitted 
to a further careful examination without revealing any flaw, after which nine 15 mm. 
cylinders were drilled out, one being broken in drilling and reground to two lengths 
of 22'2 and 25'9 mm. respectively. The eight complete cylinders were then tested 
by every available method and found to be optically perfect in every respect; the 
rotations at about 20° C. were:— 
Li . . 25’5365 La . , 25'5372 Lg length not measured L4 . . 25°‘5362 per mm. 
L, . . 25-5365 Lg . . 25-5368 Lg . . 25-5372 Lg . . 25°-5372 per mm. 
The separate values recorded above were found to be influenced by the air- 
temperature and, therefore, are only approximate. This objection does not apply to 
the group measurements which gave : 
R.J9345 . . I = 233-57 L6 a = 5!)64°-51 ajl = 25°'5361 per mm. 
Lj 456 . . . / = 231-4212 a = 5909°-78 ajl = 25°'5369 per mm. 
During the drilling of the cylinders many of the surfaces were scratched by the 
emery and subsequently re-pohshed to unequal lengths. The optical contacts were 
found to be so weak and unsatisfactory that the cylinders were cemented together 
in groups and reground to exact parallelism and equal lengths. 
The final figures after regrinding were as follows:— 
ho- 
a2o. 
<^2olho- 
/Pg • 
. 
38-4634 
982-20 
25-5360 
iR. . 
. 
41-6118 
1062-60 
25-5360 
Rg . 
50-6814 
1294-22 
25-5364 
\ R4 • 
50-6811 
1294-22 
25-5364 
R5 . 
50-6809 
1294-26 
25-5373 
■ 
56-5920 
1445-21 
25-53735 
L, . 
56-5915 
I445-20J 
25-5375 
La . 
56-5910 
1445-19 
25-53745 
. n . 
56-5915 
1445-20 
25-5374 
^ n . 
56-5914 
1445-19^ 
25-53735 
n . 
56-5921 
1445-19|- 
25-537O5 
n ■ 
56-5911 
1445-181- 
25-5373 
. L 9 • 
56-5918 
1445-21 
25-5376 
