278 DR. T. M. LOWRY: NATURAL AND MAGNETIC ROTATORY DISPERSION 
The commonest type of flaw in quartz is that due to the “twinning” of the dextro- 
and Imvo-rotatory varieties. When the intermingling is close the quartz becomes 
“feathery,” and presents the striated appearance seen in an ascending current of hot 
air ; when viewed through a Nicol prism, with light partially polarised by reflection 
from a glass surface, the striae are coloured and can be seen without difficulty. It 
is scarcely necessary to add that this fault, and other obvious flaws, were carefully 
avoided in selecting plates for the investigation. 
Plates 25 mm. in thickness were first cut from blocks of dextro- and laevo-quartz, 
and from each plate four cylinders 15 mm. in diameter were cut. On taking readings 
of the optical rotatory power of these cylinders with green mercury light, the whole of the 
four Isevo-cylinders were rejected, some because strife revealed by the monochromatic 
illumination rendered it impossible to take satisfactory readings, others because the 
readings of the cylinders (all cut from the same plate and accurately equal in length) 
not only differed from cylinder to cylinder, but also when the same cylinder was 
rotated about its own axis. The best of the four laevo-cylinders gave the readings :— 
L, . . . . 613°-17 6I3°-I7 613°-30 613°-58. Mean 613°-30. 
Of the four dextro-cylinders, one gave readings ranging over 4° and the other 
three gave :— 
Ki . . . . 613T4 613T6 613-08 613-03. Mean 613-10. 
n, ... . 613-02 613-38 613-28 613-02. „ 613-17. 
Ill .... 613-09 613-11 613-08 613-16. . „ 613-11. 
As a result of these tests, the first and last of the dextro-cylinders, which differed 
only by 0°-01, were selected as satisfactory, the two other cylinders for which figures 
are given were rejected as doubtful, the remaining four were thrown out as obviously 
faulty. 
Additional plates of dextro- and Isevo-quartz were then cut to about the same 
thickness and cylinders ent and tested in the same way by reading each in four 
positions. The figures were :— 
Ptio . . . 
. 636-80 
636-79 
636-87 
636-79. 
Mean 636-81. 
O 
1—1 
. 637-16 
636-97 
636-80 
636-16. 
55 
637-02. 
II30. . . 
. 636-81 
636-76 
636-75 
636-76. 
55 
636-77. 
II40 • • 
. 636-79 
636-75 
636-76 
636-76. 
55 
636-77. 
Lirt • 
. 640-271 
640-24 
640-27 
640-33. 
5 5 
640-28. 
LqII . 
. 640-27 
640-25 
640-29 
640-31. 
5 5 
640-29. 
L30 • 
. 640-43 
640-21 
640-23 
640-27. 
55 
640-29. 
L40 • 
. 640-24 
640-21 
640-22 
640-26. 
5 5 
640-23. 
L50 • 
. 640-34 
640-26 
640-21 
640-24. 
55 
640-26. 
Leo • • • 
Bad extinctions. 
L70 . 
. 640-19 
640-21 
640-19 
640-14. 
55 
640-18. 
Lgo 
. 640-21 
640-22 
640-23 
640-31. 
55 
640-24. 
