IN QUAKTZ OF LIGHT IN THE VISIBLE REGION OF THE SPECTRUM. 
283 
The group figures were 
Ri2345 . . . • 
232-1194 
5927-46 
25-5363 
I1'1234 .... 
181-4382 
4633-21 
25-5360 
Li256 .... 
226-3670 
5780-76 
25-5371 
1^3489 .... 
226-3654 
5780-76 
25-5373 
of the separate 
cylinders were 
exceedingly 
satisfactory, 
extreme difference in 40 pairs of readings for positive and negative half-shadow angles 
in the case of the eight leevo-cylinders, was only 0°'04 ; the extreme range of variation 
in the average values for the eight cylinders was only 0°‘02 ; the extreme variation in 
length after regrinding amounted only to O'OOll mm. and the extreme variation of 
rotatory power per millimetre to 0°'0006 per mm. 
The final values for the optical rotation of the quartz were 
Lsevo. 
Dextro. 
Before regrinding 
. . 25-5369 
25-5361 
After ,, ... 
. . 25-5371 
25-5363 
55 ... 
. . 25-5373 
25-5360 
25-5371 
25-5361 
In vi^ of the much more favourable conditions prevailing in the measurements of 
the laevo-cylinders, it was decided that these must be made the basis of the values 
finally taken, the dextro-cylinders being used mainly to balance the Isevo-cylinders in 
measurements of rotatory dispersion. The value adopted for the rotatory power of 
quartz at 20° C. for Hg 5461 is therefore 
25*5371 degrees per millimetre. 
The sixth figure has been used and is of importance in connection with the column 
of relative values given in a subsequent table for twenty-four wave-lengths in the 
visible region of the spectrum ; but it is impossible to regard it as having any absolute 
value until other equally good crystals of quartz have been found and tested and 
examined in the same way. When, however, it is noticed that the value 25°’5338 per 
mm. given by the faulty specimens used in the earlier part of the work differs only 
by 0°‘003 per mm. from the value finally adopted and that the dextro-cylinders 
(selected with some difficulty from the only sound portions of a faulty block) gave a 
value 25°'5361 per mm., only 0°’001 below the final figure, there can be little doubt 
that the fifth figure in the value 25°'5371 per mm., given by the optically-perfect 
laevo-quartz crystal, may be regarded as correct. 
o 2 
o 
