3 o8 SCIENCE PROGRESS 



has been found for the 7-rays of radium, which ignore atomic 

 structure, even in the case of the heaviest atoms. 



Logarithmic curve of transmission. — When screen and anti- 

 cathode are of the same metal, it is interesting to consider for 

 a moment the curve of transmission in which the logarithm of 

 the intensity is plotted as ordinate against thickness of screen. 

 The main portion of the curve is a straight line. It is steepened 

 in its earlier portion, and is succeeded by a region in which the 

 slope gradually diminishes as the thickness of screen increases. 

 The straight-line portion of the curve displays, of course, an 

 exponential absorption : its slope gives a measure of the 

 absorption coefficient. An explanation of the initial steepening 

 would be furnished if a certain amount of soft radiation were 

 present. This being the case, the preliminary rapid decrease 

 could be eliminated by first inserting a screen of some other 

 metal thick enough to remove all the soft rays. But on trying 

 the experiment it is found that the general shape of the curve 

 is unchanged, the kink still remains. In fact, if one gradually 

 builds up a composite screen of a number of different metals, 

 the logarithmic curve consists of a series of discontinuous steps 

 made up of an initially steep and a subsequently flat portion for 

 each metal. 



It is clear that the results are not due to the presence of any 

 soft radiation, but we can find a ready explanation of the early 

 steepness in the curve if we consider the effects of secondary 

 radiation. As is well known, when Rontgen rays encounter a 

 metal surface, secondary radiation in part softer than the 

 primary is generated in varying degree. We should therefore 

 expect that the primary radiation transmitted by any screen will 

 be accompanied by a certain amount of secondary radiation 

 from the screen itself. With thick screens none of the emerging 

 secondary radiation comes from below a certain depth, that 

 proceeding from greater depths is absorbed : thus the trans- 

 mitted primary radiation is augmented by a proportional amount 

 of secondary radiation. But for screens which are thinner than 

 this layer, the emergent secondary radiation, not having suffered 

 the full absorption, is proportionately larger in amount relative 

 to the primary. Consequently, until the screen attains a certain 

 thickness, the intensity of the transmitted beam will be relatively 

 higher, and the curve of transmission steeper than in the region 

 of thicker screens. It is apropos to remember that a similar 



