ADAMS. — TRANSMISSION OF lluNTGEN RAYS. 695 



beyond our knowledge at present. This consideration will cx])lain 

 many disagreements in the results obtained by observers using dillerent 

 instruments. 



It is simplest to suppose that the effect of transmission through 

 a metallic screen upon any one component of a heterogeneous beam 

 is independent of the presence or alDsence of the other components. 

 In other words, the effect of a given screen upon the transmission 

 of a particular sort of ray is measured by an absorption coefficient 

 which may change with the conditions of the experiment only in so 

 far as those conditions affect the ray in question. An attempt has 

 been made to test the constancy of these coefficients under changing 

 intensity of the rays, by the experiments where the dependence of 

 the ratio of transmi^-sion upon the distance between the tube and the 

 screen was investigated. These experiments are given in detail on 

 pages 6S2, 683. 



It seems very probable that in experiments so conducted the varia- 

 tion in the intensity of the rays incident on the screen, involved in 

 moving the tube, very nearly fulfils the condition that the changes 

 in the intensities of all the components of the beam shall be in one 

 ratio. Except for the absorption by the air, this ratio is doubtless 

 fixed for all the different sorts of rays by some function of the dis- 

 tances involved ; and from numerous experiments with various in- 

 struments 20 it appears that the absorption by air of Runtgen rays 

 in general is negligible for such short distances as these. Of course 

 a real disturbing factor is the changing behavior of the tube, for 

 which correction is made, as well as possible, by alternating and 

 checking readings. Granting that these sources of error have had 

 no appreciable effect, we learn from the observations that the ratios 

 of transmission of the metallic screens examined are independent of 

 the total intensity of the incident radiation, when that intensity 

 changes in such a way that the intensities of all the components 

 change in the same ratio. 



If we examine this result in the light of the conclusions already 

 reached in this paper, its practical importance will appear. It is plain 

 that if we knew in advance that all of the absorption coefficients for 

 the metallic screen and the beam in question are constant so far as 

 the intensity is concerned, the constancy of the ratio of transmission 

 would necessarily follow, provided we conclude from the data of Table IV 

 that all of the absorption coefficients of platinum are independent of the 

 ■ - — _ 



20 J. Trowbridge ami J. E. Biirbank, Amor. Jour. Sci., 157, ."96 (1800) ; 

 A. SL Jlayer, Amer. Jour. Sci., 151, 467 (1896) ; C. G. Barkla, riiil. Mag., 7, 

 655 (1904). 



