192 BELL SYSTEM TECHNICAL JOURNAL 



sound pictures are recorded and reproduced. To illustrate this point, 

 some of the acoustical problems encountered at Bell Telephone 

 Laboratories are discussed. The sudden and successive changes in 

 sound intensity level to be expected in a room during the growth and 

 decay of sound from an intermittent source are pointed out. The 

 necessity of using the more general reverberation time formula, which 

 was developed over a year ago, when dealing with comparatively 

 "dead" rooms, is indicated. One type of acoustical distortion which 

 is due to interference is discussed together with the measures necessary 

 to minimize it in sound pick-up work. These phases of acoustical 

 engineering have been selected for discussion from many which 

 confront the engineer in this field. 



An Interpretation of the Selective Photoelectric Effect front Two- 

 Component Cathodes}^ A. R. Olpin. Evidence is produced to support 

 the view that photoelectrically selective, two component cathodic 

 surfaces are crystalline in nature. Then, assuming that Fowler's 

 equation for the energy of electrons selectively transmitted through a 

 single potential valley [W = {n-h'l8md-)2 is equally valid for the 

 energy of electrons selectively transmitted through the periodic 

 sequence of valleys characteristic of the potential field within a 

 crystal, and that all of the energy of photoelectrons is acquired from 

 the incident light quanta, the wave-lengths of light to which such a 

 surface should respond selectively can be computed. Such computa- 

 tions have been made with d equal to the intern uclear distance between 

 electro-positive ions in the lattice structure of alkali metal hydride, 

 oxide and sulphide crystals. The hydride crystals belong to the 

 sodium chloride type and the oxide and sulphide crystals are sup- 

 posedly of the calcium-fluoride type. The correlation between these 

 computed values and the positions of the observed selective maxima 

 is exceptionally good. Moreover, the fact that the alkali metal 

 hydrides exhibit but one selective maximum and the oxides two or 

 three maxima is in keeping with the geometry of their respective 

 crystalline types. 



Some Physical Concepts in Theories of Plastic Flow.^^ R. L. Peek, 

 Jr., and D. A. McLean. A review is given of the considerations 

 involved in the development of theories of deformation applicable to 

 the flow of soft solids in capillary tubes and under similar steady state 

 conditions. It is pointed out that the limitation to special test 

 conditions (particularly to steady states) makes it impossible to 



^^ Phys. Rev., November 1, 1931. 

 ^^ Jour. Rheology, October, 1931. 



