Vol. 6, 1920 
PHYSICS: DUANE AND STEN STROM 
477 
a speed of revolution of over 1000 turns per second has been attained. 
This makes it possible to obtain a rotation of 45° while light is passing 
twice over the distance between stations 12 V2 miles apart; or of 90°" 
if the stations are 25 miles apart. 
It might be thought that with so small a mirror face the intensity of 
the return image would be insufficient ; but by placing the revolving mirror 
in the focus of a very long focus lens or mirror, and placing the slit source 
at a distance v = P/D, in which / is the focal length and D the distance 
between stations, the intensity is a maximum and yet a small mirror is 
sufficient to return all the light which enters the lens. This last (or the 
concave mirror which replaces it) should be as large as possible as also, 
of course, the distant mirror. 
Incidentally the eight reflections from the revolving mirror, will give 
four times as much light as a two-faced plane mirror. 
Preparations are now in progress for a preliminary test of the method 
at Mt. Wilson, where it is hoped a distance of 25 miles may be utilized. 
ON THE K SERIES OF X-RAYS 
By WilIvIAM Duank and W11.HE1.M Stenstrom 
Jefferson Physical Laboratory, Harvard University 
Communicated July 1, 1920 
In the research reported in this paper the authors have measured 
the wave-lengths of the emission lines and that of the critical absorption 
in the K series of tungsten. In order to obtain as precise values of the 
wave-lengths as possible they employed spectra of the first, second, third,, 
fourth and, in one instance, the fifth order. 
The object of the research has been to provide data for testing the 
following points: (a) The existence of a third line in the a-group; (6) 
the separation of the critical absorption from the line of shortest wave- 
length in the emission spectrum, namely the 7 line; (c) the experimental 
and theoretical relations between the various lines in the K, L, M, etc.,, 
series; (d) the relative intensities of the emission lines; and (e) the equa- 
tions for the wave-lengths that may be deduced from theories of the struc- 
ture of atoms and the mechanism of radiation. 
The X-ray spectrometer, the generating plant used to operate the X-ray 
tube, the instruments for controlling the current and voltage and the 
general method of making the measurements have been described in 
papers from this laboratory published during the last few years in the 
