EMISSION AND TRANSMISSION OF RONTGEN KAYS. 145 



The fact is clear from the table that for the same radiator \/p is relatively low 

 when screen and radiator are alike. With a radiator of copper (or any member of the 

 Cr-Zn group) the effect is very marked, and BENOIST'S transparency curve would in 

 this case be modified by the addition of a sharp maximum in the neighbourhood of 

 the atomic weight of the radiator. BARKLA and SADLER* have obtained a similar 

 result in the case of secondary Rontgen rays. With an aluminium anticathode and 

 the potential used the transparency curve would be not very far from a straight line. 



The Initial Steepening of the Logarithmic Curves. The early steeper portion of each 

 logarithmic curve of transmission when radiator and screen are alike has been noticed 

 (figs. 7, 8, and 9). The extent of the steepening depends on the material of the screen. 



An explanation which suggests itself is that the effect is due to the presence, 

 in the radiation, of a certain amount of soft rays. If this were the cause, however, we 

 should be able to eliminate the preliminary rapid decrease by placing between the 

 aluminium window of the tube and the screen a sheet of some other metal thick 

 enough to remove all the soft rays. This was tried with several metals, but always 

 with the same result. Beyond small changes in the gradients no alteration in the 

 shape of the curve was produced, and the kink still remained. In fact, if one 

 gradually builds up a composite screen of a number of different metals, the logarithmic 

 curve of transmission consists of a series of discontinuous steps made up of an initially 

 steep and a subsequently flatter 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 earlier steepness in the curves if we consider the 

 effects of secondary radiation. At every stage the primary radiation transmitted by 

 a screen is augmented by a certain amount of secondary radiation (in part softer 

 than the primary) from the screen itself. For simplicity, let us consider the case of 

 homogeneous primary rays. With thick screens none of the secondary radiation 

 emerging on the far side of the screen comes from below a certain depth of the screen : 

 that proceeding from greater depths is absorbed. Thus, in this region, the transmitted 

 primary radiation is increased by a proportional amount of secondary radiation, whose 

 presence does not conflict with an exponential law of absorption. 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 

 transmitted primary. Consequently, until the screen attains a certain thickness, the 

 intensity of the transmitted radiation will be relatively higher and the curve of 

 transmission will be steeper than in the region of thicker screens. 



This explanation is supported by the degrees of abruptness with which the 

 steeper parts of the aluminium, copper, and platinum curves (radiator and screen 

 being alike) merge into the subsequent slopes. BARKLA and SADLERf have shown 

 that with soft primary rays, aluminium emits secondary rays similar to the primary in 



* BARKLA and SADI.EE, ' Phil. Mag.,' p. 416, Sept., 1907. 

 t BARKLA and SADLER, ' Nature,' p. 344, Feb. 13, 1908. 

 VOL. CCIX. A. <J 



