150 ME. G. W. C. KATE ON THE 



is relatively low. One of the consequences of this is that the shape of BENOIST'S 

 " transparency " curve (which indicates that \/p increases with the atomic weight of 

 the screen), hesides depending on the range und degree of absorption, is largely 

 dependent on the material of the anticathode. For example, the curve is much 

 straighter for a radiator of aluminium than for one of platinum, working under the 

 same conditions. With an anticathode belonging to the chromium-zinc group the 

 transparency curve has to be modified by the addition of a sharp maximum in the 

 neighbourhood of the radiator. BAKKLA and SADLER have obtained a similar result 

 in the case of secondary Rontgen rays. 



(7) The question of the atomic weight of nickel has been discussed and an 

 explanation put forward to account for the anomalous results obtained in connection 

 with the secondary radiation from this element. 



(8) The curve of transmission, in which the thickness of screen is plotted against 

 the logarithm of the intensity, consists in general of three parts when radiator and 

 screen are of the same metal. First, with thin screens there is a relatively steep 

 portion, which for thicker screens is followed by a straight-line region : this, again, is 

 ultimately succeeded by a region in which the slope gradually diminishes with the 

 thickness of the screen. Corresponding to the straight-line portion of the curve there 

 is, of course, an exponential absorption. The extent of this region diminishes with a 

 rise in the potential on the tube. The preliminary steepening is attributed to secondary 

 radiation : in amount, it agrees with that obtained for the same metal by MCCLELLAND 

 working with the /3 rays from radium. The ultimate flattening of the curve is 

 probably due both to scattering and to the presence of hard rays. This latter region 

 may not be detectable if the potential on the tube is not too high, and the absorption 

 curve then indicates homogeneity throughout its length. 



(9) When screen and radiator have remote atomic weights, the region of expo- 

 nential absorption does not appear. The early portion of the logarithmic curve is 

 steepened by secondary radiation, but throughout the whole region the transmission 

 is one in which the coefficient of absorption steadily diminishes as the thickness of 

 screen increases. This result is probably brought about in the early stages chiefly by 

 scattering, and in the later stages by the heterogeneity of the beam. 



In conclusion, it may be remarked, the present research shows that the terms hard 

 and soft rays should be confined to comparisons with screens of the same metal. 



It gives me pleasure to thank Prof. THOMSON for his interest in this investigation. 

 I wish also to express my indebtedness to Mr. E. EVERETT for his timely and ready 

 assistance on occasion. 



Note on the Use of Tilted Electroscopes. As a good deal of time may easily be 

 spent in adjusting a tilted electroscope to sensitiveness, it may be permissible to 

 mention one or two points in connection with it which I have not seen dealt with 

 elsewhere. 



