92 J. M. Adams — Spectrum of the Rontgen Rays. 



ing the band into halves lengthwise, and photographing it 

 while one-half was covered by a sheet of copper 0"0044 cm thick, 

 the other half being uncovered. Figure 2 shows a photograph 

 thus obtained. The width of the band was limited by brass 

 strips with bevelled edges about 0*3 cm apart. The Rontgen 

 rays produced by the least deviated cathode particles are at A, 

 those produced by the most deviated cathode particles at B. 

 The difference in absorbing power of the copper at the two 

 ends of the band is plain. The variations in intensity along 

 the uncovered half of the spectrum give a rough idea of the 

 distribution of the different kinds of rays. 



The existence of relatively selective absorption in the case 

 of one pair of metals, aluminium and silver, was shown by 

 photographing one-half of the spectrum through a sheet of 

 one metal, the other half through a sheet of the other. The 

 aluminium was 016 cm thick, the silver O002 cm . Figure 3 

 shows a set of these photographs. In that figure CD is a com- 

 parison spectrum, photographed without the interposition of 

 any metal. In the photograph EF, the silver was on the left 

 and the aluminium on the right. The thicknesses of the two 

 sheets were so chosen that the rays at E, corresponding to the 

 least deviated cathode particles, were equally absorbed in 

 them. Under these circumstances, the rays at F were trans- 

 mitted by the silver in much greater quantity than by the 

 aluminium. To show that this effect was not to be explained 

 by a difference in. thickness of either sheet at different points 

 along the spectrum, the sheets were turned in their own plane 

 through two right angles, and the photograph GH was then 

 made, showing the effect unchanged. Secondary radiation 

 from the sheets was not present in appreciable quantity at the 

 photographic plate ; for if it had been, it would have resulted 

 in a blurring of the boundaries of the shadows on the plate, 

 the distance between the latter and the sheets being about 2 cms . 



Another pair of metals, aluminium and tin, has been found 

 to show relatively selective absorption to some extent, but the 

 effect has not yet been obtained sufficiently well marked for 

 reproduction. 



The conclusions to be drawn from these experiments are 

 the following: 



(1) The beam of Rontgen rays from a focus tube which 

 yields a magnetic cathode spectrum is heterogeneous. 



(2) A metallic sheet shows selective absorption of the dif- 

 ferent rays. 



(3) This selective absorption does not follow the same law 

 in all metals : in certain pairs of metals the absorption is rela- 

 tively selective. 



Jefferson Physical Laboratory, 



Harvard Univ., Cambridge, Mass. 



