468 Scientific Proceedings. Royal Dublin Society. 



the action of the 7 rays on any screen used, and (4) secondary |3 rays 

 produced in the tissues by the passage of the 7 rays. 



Tlie a rays are completely stopped by less than 0-1 mm. of flesh, or 

 about O'Oo mm. of glass. This being much less than the usual thickness 

 of wall of an emanation tube, they produce no effect with the common 

 modes of treatment. If, however, emanation solution be injected, the effect 

 of the a rays must far outweigh that of the others in all regions to which 

 the solution finds access, as their relatively large energy and great 

 absorbability cause, within their range, an intensity of ionisation about 

 100 times that due to the |3 rays. 



The primary /3 rays, as may be inferred from the curves shown in fig. 2, 

 are completely absorbed by about 1'5 cms. of flesh, or 1'5 mm. of brass. 

 Owing to this comparatiA^ely rapid absorption, they produce relatively 

 intense ionisation, that due to unscreened |3 rays being about 100 times 

 that due indirectly to the 7 rays from the same source. 



Secondary /j rays, produced by the passage of 7 rays through ordinary 

 matter, are similar in ionising properties to the primary /3 rays emitted 

 by the radioactive source. They are, however, considerably less penetrating 

 than the hardest of the primary /3 rays. The velocity, and hence the 

 penetrating power of secondary /3 rays, increases with the hardness of 

 the 7 radiation to which they are due, so that the secondary j3 rays produced 

 by 7 rays are much more penetrating than those due to X rays. 



If a metal screen is interposed between the source and the flesh, the 

 surface of the latter is exposed to the secondary /3 radiation from the screen, 

 but is deficient in secondary radiation from the flesh as compared with a 

 point deeper down. 



The effect at a point in the tissue, then, varies with depth owing to the 

 combined effect of the following causes : — 



(1) Absorption of primary /3 rays in the screen and the tissue. 



(2) Absorption of secondary /3 rays due to the screen. 



(3) Initial increase with depth of the secondary /3 rays due to the tissue. 



(4) Absorption of 7 rays, and consequent reduction of secondary )3 rays. 



(5) Geometrical effect of distance. 



The joint result of effects (1) to (4) has been estimated by experiments 



with an electroscope at a fixed distance from the source, and approximate 



calculations then made to include the effect of distance in some typical 



cases. 



Apparatus. 



The electroscope used is shown in section in fig. 1, which also shows 

 the emanation tube mounted on a wooden block at a fixed distance below 



