692 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



The means of the ratios of transmission for each pair of metals 

 follow : 



Copper-aluminium, 0.424 

 Tin-aluminium, 0.567 

 Aluminium-silver, 0.489 

 Copper-tin, 0.336 

 Tin-silver, 0.368 



Aluminium -copper, 0.430 

 Aluminium-tin, 0.560 

 Silver-aluminium, 0.489 

 Tin-copper, 0.331 

 Silver-tin, 0.365 



The thicknesses used were : Copper, 0.0044 cm. 



Tin, 0.0012 cm. 

 Silver, 0.0019 cm. 

 Aluminium, 0.050 cm. 



TABLE XI. 

 Check on Correctness of Procedure in Experiment of Table X. 



V. Discussion of Experimental Results. 



The qualitative law which connects the thickness of a screen with the 

 transmission through it, as stated and illustrated on pages 680, 681, 

 was first enunciated by Rontgen.^^ He explained the phenomenon by 

 assuming that the beam emitted by the tube consists of a mixture of 

 different kinds of rays, some of which have more penetrating power 

 than others. If such a beam were passed through a plate of any sub- 

 stance, the less penetrating rays would suffer absorption to a greater 

 extent than the more penetrating ones, and the penetrating power 

 of the emergent beam would be on the whole greater than that of 



the 



original beam. 



This explanation is a satisfactory one so far as 



« W. C. ROntgen, Ann. d. Phys., 64, IS (1898). 



