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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



out rotation. When M 1 is slightly behind M 2 , as shown, a wave train 

 reflected from the upper mirror has a longer path to follow from 

 sender to receiver than the other half of the same train reflected by 

 the lower mirror. It is thus delayed, and when the two halves of the 

 beam are reunited at the receiver, the separated wave trains may no 

 longer match. By varying the distance by which the plane of the 

 mirror M, is behind that of the mirror M,, wave crests in one beam 

 can be made to fall on the earlier wave crests of the other, and thus 



B 



Fig. 4. — Sketch illustrating the formation of a relatively long wave train from a short 



electromagnetic pulse 



the two beams reenforce each other. On the other hand, wave crests 

 of one can be brought together with the wave troughs of the other, 

 and the two made to weaken each other. Observed positions of the 

 mirror M. 1 for reinforcements and neutralizations thus give a direct 

 means for measuring the wave length of the radiation. 



When oscillator and receiver are strictly in tune, interference 

 curves like those shown in Figure 3 are obtained. In these wave 

 diagrams, vertical distances from to 100 represent radiant inten- 

 sities as shown by the receiver. Horizontal distances to the right 



