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The amount of gain, its onset at a drift field high enough for the electron drift velocity to equal the elastic wave velocity, and the observed decrease in gain upon changing the resistivity from the optimum value, are all similar to the case of bulk wave amplification. It should be noted that in these experiments the drift field existed only between the transducers, whereas the illumination was uniform both between and beneath the transducers, producing some constant attenuation at the transducers. A higher gain per unit length would be observed if the illumination or crystal conductivity were altered so as to eliminate this loss beneath the transducers. Surface-wave propagation has been observed in CdS crystals having their c axes either normal or parallel to the surface plane. For both orientations, a surface-wave velocity of (1.73 ± 0.05) X lOS em/sec was inferred from the period of the transducer electrodes (2.20 X 10-2 em) and the measured frequency which maximized the received pulse amplitude. This velocity agrees well with the value 1.70 X lOS em/sec calculated for CdS from an expression derived by Stoneler which is applicable to the firstmentioned orientation. The velocity of surface waves in CdS is only slightly smaller than the velocity of transverse bulk waves. That we have indeed dealt here with surface waves rather than bulk transverse waves is shown by the following: (a) velocity measurements in experiments with crystalline quartz (in which the velocities of bulk and surface waves are weJI separated) showed that the electrodes function as surface wave transducers; (b) on quartz, excellent transmission was observed from one electrode transducer to a metallic comb surface-wave transducer as described by Viktorov,4 and Arzt and Dransfeld;5 (c) a drop of acetone or alcohol on the CdS surface produced apparently complete absorption of the elastic pulse; (d) probing the CdS crystal with a narrow beam of light directed parallel to the plane surface produced a variable attenuation of the received pulse which showed that the wave amplitude was high near the surface and that it was negligible at large depths. This work was supported in part by the U. S. Army Research Office Durham, under Grant

TWO‐BEAM INTERFEROMETRY BY SUCCESSIVE RECORDING OF INTENSITIES IN A SINGLE HOLOGRAM