1906-7.] Optical Rotation of Two Salts. 275 
ment of H and He vacuum tubes, and by the use of Na, Sr, Th, and Li salts 
in a Bunsen flame. The results were as follows : — 
Reading of telescope scale, 
154-5 
156 
157 
158 
159 
160 
Mean wave length, 
455 
487 
515 
550-5 
598-5 
666 
Longest wave length in 
the 1 
transmitted light, . 
.1 
463 
496’5 
525-3 
565 
628 
745 
Shortest wave length in 
the I 
transmitted light, . 
.} 
447 
476*5 
500 
530 
573 
637 
The consequent impurity, . 
16 
20 
25-3 
35 
55 
108 
The settings of the Nicol prism of the instrument were automatically 
recorded by the author’s photographic recorder, to be subsequently described, 
2000 
1600 
-1200 
l§ 
= O 
a a; 
w 
800 
400 
50 
65 Wave-Length 60 
Fig. 2. 
66 
the expense of which was met by a grant from the Carnegie Trustees : a 
plan which was found to greatly facilitate the work. The zero positions of 
the Nicol were ascertained with the polarimeter tube in place and filled 
with distilled water at 25° C., and thereafter the end discs of the tube were 
