472 BELL SYSTEM TECUXICAL JOURNAL 



to the lens Lj which merges ilicin in a coniinon fixus at .1/, where 

 the receiver stands.'' 



The intensity at M depends, by virtue of the principle of inter- 

 ference of periodic waves in its simplest conceivable application, on 

 the ratio of the distance between the planes of the two niirmrs to the 



L, 



v~- 



t 



c5; 



Diagram for wave-length measurements. 



Fig. ^ — I'.illi 1)1 tht Radiation from Oscillator to Receiver. (Physical Rifinv) 



Fig. 4— Photograph of the Hollzmann Intcrfcroiiiotcr. (Physical Review) 



waxelength of the rays. // at .\f there were a receiver of which the 

 reading was perfectly proportional to the amplitude of the vibration 

 at M and if the original wave-train were perfectly sinusoidal and 



' In the sketch S is a semi-transparent mirror (glass ebonite, or cardboard) which 

 reflects a part of the beam to a lens L-, and focus C where its intensity can be measured 

 at the same moment as the intensity at M. The variability of the output of the 

 source makes this control indispensable 



