storage and subtraction. At the Institute of Optical Research for 
several years a system for vibration analysis has been in use which 
provides very simple means for electronically processing the inter- 
ferogram: the four terms of a holographic recording are separated in 
bandwidth due to a double slit aperture of the imaging lens with the 
reference source in the center of the aperture [3]. 
  
  
  
An elliptic membrane vibrating in a combination mode. By touching the 
membran one can observe on the TV-screen when one or another of the 
two modes is damped out.