412 
PHYSICS: C. BARUS 
TABLE 2 
108 
X 
PR1£QUE^CY 
DIFFER- 
ENCE 
INT. 
108 
X 
FREQUENCY 
DIFFER- 
ENCE 
6167.4 
8 
16214.29 
1038.41 
4366.5 
4 
22901.64 
1033.25 
5796.2 
5 
17252.70 
4178.0 
6 
23934.89 
5811.7 
3 
17206.67 
1034.35 
5907.4 
2 
16927.92 
2017.57 
5482.15 
6 
18241.02 
5278.3 
1 
18945.49 
A preliminary attempt at a Rydberg formula gives the following re- 
sults for the first line of the three classes of triplets : 
Principal series, n = 44349.0 109675 
First subordinate series, n = 25236. 8 — 
Second subordinate series, n = 25153.0 
{m + 1.0855)2 
109675 
(m + .8062)2 
109675 
{m + .6196)2 
It is very probable that further investigation will modify these for- 
mulae, but the existence of the triplets seems well established, and the 
fact that alternate elements in the chemical table show a spectroscopic 
relation is suggestive for further work along this line. 
' Runge, C, and Precht, J., Phil. Mag., London, 5, 1903, (476). 
2 Ives, H. E., and Stuhlmann, O., Physia. Rev., Ithaca, N. Y., (Ser. 2), 5, 1915, (368). 
' Kayser, H., Handbuch der Spedroscopie, vol. 6, p. 325. 
THE MEASUREMENT OF SMALL ANGLES BY DISPLACEMENT 
INTERFEROMETRY ' 
By Carl Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITY^ 
Communicated April 21. 1917 
Parallel rays retracing their path. — The following method was devised 
with a view to the micrometric measurement of angles. It is to be used 
below in connect'on with an electrometer for reading microvolts. An 
interference method of a different kind for measuring small angles was 
developed some time since and used at length in connection with the 
deviation of the horizontal pendulum. ^ Again the electrometer was 
treated in different ways^ by the aid of the displacement interferometer. 
The present method, however, will differ from all of these. In figure 1, 
L is a horizontal beam of white light from a collimator. After passing 
