RONTGEN RAYS 307 



the same or any other metal : the result has usually been 

 ascribed to the selecting or sifting out of certain rays from the 

 original heterogeneous bundle. Walter, in 1905, noticed the 

 effect to be marked in the case of the metals of the silver group. 1 

 His investigations led him to believe that the rays are actually 

 transformed by the screen in such a manner as to render them 

 more penetrating for any subsequent material. Sagnac claims 

 that his experiments on the reversing of composite screens 

 point to a similar conclusion, though his results are not 

 supported by Adams (1907). The latter, in his experiments 

 mentioned above (p. 302), obtained both photographic and 

 thermal evidence of the special transparency of different metals 

 to certain rays. Haga (1907) has also noticed a similar selective 

 absorption in the case of carbon and ebonite. 



It may be pointed out that the statement above as to the 

 increased penetrating power of a beam of rays after transmission 

 is not perfectly general ; for Kaye recently showed that when 

 screens and anticathode are of the same metal, an exponential 

 absorption is exhibited over a considerable region. This sign 

 of homogeneity disappears when screen and anticathode are 

 unlike, and is replaced by the type of absorption usual with 

 Rontgen rays, that is, one in which the coefficient of absorption 

 shows a gradual diminution as the screen thickness increases. 

 It is thus unsafe to conclude that such a type of transmission 

 is indicative of heterogeneity : a screen of another metal may 

 yield quite a different conclusion. 



For screens of like metal to the radiator the rays show 

 special penetrating power, so that not only is the transmission 

 exponential but selective. The effect becomes less and less 

 marked as the screens increase in thickness, and harder and 

 harder rays are concerned. This is in accordance with what 



1 Prof. Walter used Benoist's " Scale of Hardness " in some of his experiments. 

 This consists of a series of aluminium plates of a variety of thicknesses, sur- 

 rounding a central silver plate. By a photographic or fluoroscopic method a 

 match is obtained between the silver plate and one of the aluminium plates, when 

 the rays to be tested are sent through the scale : the thickness of this aluminium 

 plate is taken as giving a measure of the hardness of the rays. The instrument 

 has its practical value on account of its rapidity, but the interpretation of its results 

 has led some observers into confusion. In quantitative measurements of hardness 

 it is much better to employ a method which will note the changes of intensity 

 that accompany the use of screens of one metal (preferably one of low atomic 

 weight) alone. 



