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XXVIII. A Difference in the Photoelectric Effect caused by 

 Incident and Emergent Light. By Otto Stuhlmann, Jr., 

 A.B.j Experimental Science Fellow, Princeton University*. 



Introduction. 



REGENT investigations have shown that the ionization 

 produced by the secondary rays arising from a thin metal 

 plate traversed normally by a primary beam of 7, Rontgen, 

 or j3 rays, is greater on the emergent than on the incident 

 side. 



\V. H. Bragg f, in his work on the nature of: the 7-rays 

 and Rontgen rays, showed that it' 7-rays pass through a thin 

 plate so that the absorption is negligible, the amount of 

 emergent radiation is greater than the incident. This re- 

 markable want of symmetry he points to as appearing fatal 

 to the aether pulse theory of 7-rays and, from their many 

 points of similarity, of the Rontgen rays also. In a later 

 paper on the nature of y-rays W. H. Bragg and J. P. V. 

 MadsenJ also show that this want of symmetry holds for both 

 7 and /3-rays. This lack of symmetry for secondary Rontgen 

 rays was also discovered by W. H. Bragg and J. L. Grlasson§. 

 They showed that this want of symmetry was in general 

 more pronounced for the softer rays than for hard rays. 

 That the proportion of emergent to incident radiation differed 

 considerably for the different radiators, but was much the 

 same for different thicknesses of screen, except tbat the pro- 

 portion tended to increase slightly as the screen was made 

 thicker ; and the tendency was most pronounced in the case 

 of those metals which gave out a quantity of soft secondary 

 radiation, the emergent secondary rays being generally in 

 excess of the incident. 



The present experiments were made to see if there was 

 any difference in the photoelectric effect caused by the 

 incident light and the light which emerged after passage 

 through a thin metal film. 



' Apparatus. 



Thin films of platinum were prepared by sputtering in 

 vacuo from a platinum cathode on to quartz plates 1 mm. 

 thick. Two plates were sputtered simultaneously so as to 

 insure the same thickness for both films. These were now 

 mounted in the centre of two similar brass cylinders A and B 



* Communicated by Prof. O. W. Richardson. 



+ Bragg-, ' Nature,' lxxvii. pp. 270-271, Jan. 23, 1908. 



t Bragg and Madsen, Phil. Mag. xvi. pp. 918-939, Dec. 1908. 



§ Bragg and Glasson, Phil. Mag. xvii. pp. 855-864, June 1909. 



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