568 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



Fig. 5 



-A strip prism placed before the lens of Fig. 3 tilts the focussed beam downward. 



/= 9KC. 



iffli 



Fig. 6 — Adding a signal of constant phase to the signal picked up by the scanning micro- 

 phone delineates the position of the wave crests as they progress outward toward 

 the right. (These are not standing waves.) / = 9 KC. 



