694 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The experiments of the present research upon the effect of trans- 

 mission through one metal on penetrating power for another were 

 undertaken for the purpose of confirming or disproving the necessity 

 of such a theory of transformation. The results which are summarized 

 on page 688 are in agreement with the general statement which Walter 

 has deduced from his fluoroscopic work, so far as they go ; but the re- 

 sult of the experiments with silver and aluminium, given on page G90, 

 is contradictory to that statement, and indicates that relatively selec- 

 tive absorption among metals may exist. 



In view of the doubt thus cast upon the necessity of a theory of trans- 

 formation, direct experimental evidence of transformation was sought 

 by means of measurements of transmission through a screen composed 

 of two pieces of different metals. These measurements are described 

 on pages 691, 692. It is to be expected that if transformation occurs 

 in the metals, the effect of the second metal upon rays transformed 

 by the first will not be quantitatively the same as the effect of the first 

 metal upon rays transformed by the second. In other words, the trans- 

 formation theory leads ua to expect that the ratio of transmission of a 

 two-piece screen will depend upon the order of the pieces ; and the ab- 

 sence of this dependence, experimentally shown, is evidence of the 

 absence of transformation. 



With the possibility of transformation in transmission thus removed, 

 and with experimental evidence ^^ showing that any effect of the sur- 

 faces of the metal upon transmission is small, the only conceivable 

 action produced upon a beam of rays by transmission through a 

 metallic screen is an absorbing action. To explain the phenomenon 

 observed by Rontgen with different thicknesses of the same metal, 

 we must suppose, with him, that the rays from a tube are hetero- 

 geneous, and that different kinds of rays are differently absorbed in 

 any one metal. To explain Walter's apparent reduction of penetrating 

 power by transmission in certain cases, we must suppose that rays 

 which are more penetrating for some metals are less penetrating for 

 others, — ■ that is, that metals show relatively selective absorption, and 

 that the apparent reduction of penetrating power by transmission 

 through silver, etc., is a real reduction of penetrating power with 

 respect to aluminium. In judging of the significance of Walter's 

 fluoroscopic work, as in fact of all work on R(>ntgen rays, it must be 

 borne in mind that the selectivity of the absorbing media of the 

 instruments themselves must greatly affect the magnitude of their 

 indications, and that an exact interpretation of those indications is 



" See Tables V-VII and Figure 5. 



