144 BELL SYSTEM TECHNICAL JOURNAL 



gested that such a nucleation and growth reorientation phenomenon is re- 

 sponsible for self-recovering in cold-worked metals. 



The formation of CuAU precipitate particles is demonstrated with both 

 electron micrographs and diffraction patterns. A fine lamellar structure found 

 in the quenched Al-4 per cent Cu alloy is at present unexplained. 



Path-Length Microwave Lenses?] Winston E. Kock. Lens antennas for 

 microwave applications are described which produce a focusing effect by 

 physically increasing the path lengths, compared to free space, of radio 

 waves passing through the lens. This is accompUshed by means of baffle 

 plates which extend parallel to the magnetic vector, and which are either 

 tilted or bent into serpentine shape so as to force the waves to travel the 

 longer-inclined or serpentine path. The three-dimensional contour of the 

 plate array is shaped to correspond to a convex lens. The advantages over 

 previous metallic lenses are: broader band performance, greater simplicity, 

 and less severe tolerances. 



Refracting Sound Waves. ^] Winston E. Kock and F. K. Harvey. 

 Structures are described which refract and focus sound waves. They are 

 similar in principle to certain recently developed electromagnetic wave lenses 

 in that they consist of arrays of obstacles which are small compared to the 

 wave-length. These obstacles increase the effective density of the medium 

 and thus effect a reduced propagation velocity of sound waves passing 

 through the array. This reduced velocity is synonymous with refractive 

 power so that lenses and prisms can be designed. When the obstacles ap- 

 proach a half wave-length in size, the refractive index varies with wave- 

 length and prisms then cause a dispersion of the waves (sound spectrum 

 analyzer). Path length delay type lenses for focusing sound waves are also 

 described. A diverging lens is discussed which produces a more uniform 

 angular distribution of high frequencies from a loud speaker. 



Double-Stream Amplifiers?] J. R. Pierce. This paper presents expressions 

 useful in evaluating the gain of a double-stream amplifier having thin con- 

 centric electron streams of different velocity and input and output gaps 

 across which both streams pass. 



Direct Voltage Performance Test for Capacitor Paper }^] H. A. Sauer and 

 D. A. McLean. Performance of capacitors on accelerated life test may vary 

 over a wide range depending upon the capacitor paper used. Indeed, at 

 present a life test appears to be the only practical means for evaluating 



■^ Proc. I. R. E., V. 37, pp. 852-85.S, Augu?t 1949. 

 ^ Acous. Soc. Amer. Jour., v. 21, pp. 471-481, September 1949. 

 ' Proc. I. R. E., V. 37, pp. 980-985, Septeml)er 1949. 

 '« Proc. I. R. E., V. 37, pp. 927-931, August 1949. 



t A reprint of this article may be obtained by writing to the Editor of the Bell System 

 Technical Journal. 



