Physics. — “A Theory of a Method for the Derivation of the 
Distribution of Energy m a Narrow Spectrum Region from 
the Distribution of Energy, Observed in an Interferometer’. 
By K. F. Niessen. (Communicated by Prof. W. H. Junius.) 
(Communicated in the meeting of May 31, 1919). 
Micuetson has pointed out the direction in which it should he 
tried to find such a method'). He first considers two absolutely 
monochromatic beams, which he causes to interfere with a difference 
of phase. If this takes place in the interferometer of MicHELson, in 
which the plane of reference may lie at a distance / before the movable 
mirror, the difference of phase of the rays that have struck the 
2l 
mirrors at right angles is ni X 20 (or 4rlm, when we work with the 
number of waves m, by which we understand the number of waves 
in the unity of length). The ocular is adjusted for infinity, and in 
the middle of the field of vision (for there the rays striking at right 
angles interfere) an intensity J/(/) is observed, which can be calculated 
by the aid of the formula ’): 
JSA eae ad, cos An ln oenen ee 
from the equal intensities J, of the interfering beams. An arbitrary 
beam of light may be thought to be divided into an infinite number 
of absolutely monochromatic beams of the intensity : 
Jr WA da Or == in) Gee ee 
when we use the number of waves m. Ag intensities which are due 
to different frequencies join scalarly, this light admitted to the 
interferometer of MrcuersoN, will give rise in the point under consi- 
deration to the intensity J(/), given by: 
1) Phil. Mag. (5) 31, pag. 338, 1891. 
4) Drupr, Lehrb. der Optik, p. 123. 
