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LXXVIII. An Apparent Softening of Rontgen Rays in Trans- 

 mission through Matter. By Charles A. Sadler, D.Sc, 

 Oliver Lodge Fellow, and Alfred I. Steven, M.A., B.Sc, 

 Lecturer in Physics, University of Liverpool*. 



A LARGE proportion of the rajs emitted by the anti- 

 cathode oE an ordinary X-ray tube must of necessity 

 be absorbed in their passage through the glass walls, and 

 consequently the nature o£ the emergent radiation is some- 

 what modified. A recognition of this fact has led various 

 investigators to place a thin aluminium window in the walls 

 of the tube, and to examine the nature of the radiation pro- 

 ceeding through it. In particular, Kayef found that, with 

 such a bulb, the amount of the radiation emitted for a given 

 potential difference varied with the nature of the anticathode 

 used, and also gave data which indicated that in some cases 

 a fairly homogeneous beam was emitted. Subsequently it 

 was pointed out that these beams were largely composed of 

 the homogeneous radiation characteristic of the particular 

 metal used as anticathode, but at the same time there was 

 also present a certain amount of scattered radiation. 



If a piece of glass, 1 mm. thick, were placed in the path 

 of such a beam, it would practically absorb the whole of the 

 homogeneous radiation together with the softer constituents 

 of the scattered, the remainder being similar in character to 

 the radiation from an ordinary bulb. Should the characteristic 

 radiation, however, be very easily absorbed, and of no great 

 intensity, a comparatively thin layer of glass or other sub- 

 stance would suffice to cut out this portion, but the beam 

 would still contain components which are much softer than 

 those present under ordinary circumstances. 



If these conditions can be experimentally realized, it is 

 obvious that the issuing beam approximates much more 

 closely to the scattered radiation as it leaves the anticathode 

 itself. Now, as the characteristic radiation of aluminium is 

 known to be very soft and of feeble intensity compared with 

 the scattered radiation, it seemed desirable to investigate the 

 nature of the radiation proceeding from a bulb having a thin 

 aluminium window and fitted with an aluminium anticathode. 

 The scope of the inquiry was limited to a measurement of 

 the penetrating power and heterogeneity of the rays emitted 

 at different stages of exhaustion, but here an unexpected 

 difficulty arose. When the beam had reached a certain 

 penetrating power, it appeared to become softer on cutting 



* Communicated by the Authors. The expenses of this research have 

 been partially defrayed by a Government grant through the Royal Society, 

 f Phil. Trans. A. ccix. pp. 128-151. 



