ABSTRACTS OF TECHNICAL PAPERS . 635 



diameters; beyond this distance it decreases steadily. The intense 

 field close to the surface accounts for the decreased work function 

 while the reverse field farther out accounts for the poor saturation at 

 ordinary applied potentials. 



The photo-electric long wave-length limit should be shifted toward 

 the red by applied fields. This shift should be particularly noticeable 

 for composite surfaces. 



Direct Determination of Rubber in Soft Vulcanized Rubber.^ A. R. 

 Kemp, W. S. Bishop, and T. J. Lackner. A modification of the Wijs 

 method is shown to be suitable for determining the rubber content of 

 vulcanized rubber. A procedure for the direct determination of sulfur 

 combined with rubber is also outlined, and the effect of compounding 

 ingredients is shown. 



Results of analyses of four reclaimed rubbers by the proposed and 

 difference methods are given for comparison. 



Photomicrography and Its Application to Mechanical Engineering.^ 

 Francis F. Lucas. This paper was presented at the annual meeting 

 of the American Society of Mechanical Engineers during the week of 

 December fifth, 1927. It discusses the difference between magnifica- 

 tion and resolution and stresses the difficulties in obtaining clear-cut 

 photomicrographs at high magnifications. Until comparatively re- 

 cently 1500 diameters was thought to be the limit. 



The ultra-violet microscope should, theoretically, give about double 

 the resolution of one using visible light because of the shorter wave- 

 length, but until recently this has not been the case. The paper 

 explains a mechanical focusing method used in Bell Telephone Labora- 

 tories by which a series of photographs are taken with a change in 

 focus of one sixteenth micron between successive exposures. A typical 

 set of four successive exposures at 1800 magnifications is shown. 



The photomicrography of steel is gone into at some length, several 

 photographs at 3500 magnifications being shown in illustration. 

 Special importance is laid on the preparation of samples as well as on 

 careful focusing. 



1 Industrial and Engineering Chemistry, Vol. 20, No. 4, April 1928, pp. 427-429. 



2 Mechanical Engineering, Vol. 50, No. 3, pp. 205-212, March 1928. 



