J. M. Adams — Transmission of Rontgen Rays. 377 



selectivity of the absorbing media of the instruments them- 

 selves must greatly affect the magnitude of their indications, 

 and that an exact interpretation of those indications is beyond 

 our knowledge at present. This consideration will explain 

 many disagreements in the results obtained by observers using 

 different instruments. 



It is simplest to suppose that the effect of transmission 

 through a metallic sheet upon any one component of a hetero- 

 geneous beam is independent of the presence or absence of the 

 •other components. In other words, the effect of a given sheet 

 upon the transmission of a particular sort of ray is measured 

 by an absorption coefficient which may change with the con- 

 ditions 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 per- 

 centage reduction of deflection upon the distance between the 

 tube and the sheet was investigated. 



It seems very probable that in experiments so conducted the 

 variation in the intensity of the rays incident on the sheet, 

 involved in moving the tube, very nearly fulfills 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 distances involved; and from 

 numerous experiments with various instruments* it appears 

 that the absorption by air of Rontgen rays in general is negli- 

 gible for such short distances as these. Of course a real dis- 

 turbing 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 percentage reductions of deflections, clue to 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 sheet and the beam in 

 question are constant so far as intensity is concerned, the con- 

 stancy of the percentage reduction of deflection would neces- 

 sarily follow, provided we grant that all of the absorption 

 coefficients of platinum are independent of the intensity. But 



* J. Trowbridge and J. E. Burbank, tbis Journal, vii, p. 396, 1899 ; A. M. 

 Mayer, ibid., i, p. 467, 1896 ; C. G. Barkla, Pbil. Mag. vii, p. 555, 1904. 



