134 



MR G. W. C. KAYE ON THE 



that, as the atomic weight of the metal of the screen increases, the harder are the rays 

 for which it shows selective transmission. 



General Comparison of Screens. -The following selected values of the relative 

 radiations will give a notion as to the degree and extent of the selective transmission 

 shown by different screens under the same conditions. As before, the radiations are 

 taken relative to that of platinum. 



Apart from the points already mentioned, the most interesting feature is the 

 nearness of the radiation values of nickel and copper and the remoteness of those of 

 nickel and iron when the thinner of each pair of metal screens is in use. This is 

 especially apparent with both thicknesses of the iron, nickel, and copper screens. 

 BARKLA and SADLER* working with secondary Rontgen rays have obtained similar 

 results and claim a higher value for the atomic weight of nickel than the one usually 

 accepted. It should, however, be noticed that with the thicker screens, especially 

 those of aluminium and platinum (and it may be added silver and tin), there is a 

 distinct tendency for nickel to move into a place justified by its accepted atomic 

 weight. It was hoped that light would be thrown on the subject by using a cobalt 

 anticathode, but unfortunately the two samples of cobalt used turned out to be 

 unsatisfactory and gave much too low a radiation value for all screens. The question 

 is referred to later (p. 139). 



The values obtained with silver and tin screens resemble those obtained with 

 platinum screens. The very large augmentation in the transmitted radiation when 

 screen and radiator are alike is confined to the metals of the chromium-zinc group. 

 For other metals the effect is much smaller. 



The general conclusion may be drawn that of all the screens tried aluminium shows 



BARKLA and SADLER, ' Phil. Mag.,' p. 409, Sept., 1907. 



