Physics. — “Measurements on the Intensity of Spectrum Lines by 
the Aid of the Echelon’. By Dr. H. C. Burerr and P. H. 
VAN Cittert. (Communicated by Prof. W. H. Junius). 
(Communicated at the meeting of September 25, 1920.) 
1. Introduction. When determining the intensities of spectrum 
lines, one is confronted by the following complication : what is directly 
observed is the relation of the intensities of the lines which exists 
at the place where the examined spectrum is formed by the spectrnm 
apparatus used. In general, however, this relation is not the same 
as the relation of the intensities of the lines in the light emitted 
by the examined source of light. In the echelon this is even far 
from being the case for very small differences of wave-length. 
When the intensity of the light that traverses the echelon in the 
direction of the optical axis, is /,, the intensity of the light 
leaving at an exit angle « with the axis is theoretically *) given by: 
oe eae 
fay ee 
(rie) 
(2 = wave-length, o = width of a step). 
The differences in direction of exit may have been caused both 
by a difference in wave-length and by a difference in position of 
the echelon, provided the echelon is placed about parallel to the 
optical axis. In fig. 1 the relation between intensity and position (i.e. 
angle a) of a spectrum line is graphically represented. At an angle of: 
a 
Dese . 
the intensity becomes zero, and assumes only small values outside 
this interval. The distance between two orders also amounts to 
0 , 
À 
«a. = — so that at the utmost two orders of one line in the central 
6 
part of the curve (fig. 1) can be observed with pretty great intensity. 
As appears directly from the figure, it may happen that the intenser 
1) Enc. d. Math. Wiss., Band Physik V, 21, 389. 
BALy-WaAcHSMUTH, Spektroskopie, 1908, 137. 
