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[ 37 ] 



III. On the Asymmetry in the Distribution of Secondary 

 Cathode Rays produced by X-Rays ; and its Dependence on 

 the Penetrating Power of the Exciting Rays. By C. D. 

 Cooksey, Ph.D., Instructor in Physics, Sheffield Scientific 

 School, Yale University*. 



IT has been shown by Bragg and Madsen f that the 

 amount of secondary /3 radiation excited in various 

 solids by y-rays is not symmetrically distributed about a 

 plane perpendicular to the direction of propagation of the 

 Y-rays, but that the /3 radiation which comes from the side 

 from which the Y-rays emerge is greater than that coming 

 from the side on which the Y~ ra y s are incident. The same 

 effect is observed, though to a less degree, in the case of 

 secondary X-rays J, and the secondary cathode rays pro- 

 duced by X-rays §, and those produced by ultra-violet light || . 

 This lack of symmetry is less for soft Y-rays than for hard 

 and still less for X-rays, which are usually considered as 

 very soft Y~rays. The order of magnitude of the ratio of 

 emergence to incidence radiation ranges all the way from 

 20 : 1 down to unity, depending on the nature of the radia- 

 tions used and the substance in which the secondary rays 

 are excited. 



Since I first showed that this effect was true for the 

 cathode rays produced by X-rays, I have been experimenting 

 with a view to find how the ratio of emergence to incidence 

 radiation depends on the hardness of the exciting rays. But 

 with these rays the effect is so small at best that the 

 variations which might be produced by the widest possible 

 variation in hardness of the primary are not likely to be 

 much greater than the experimental errors always inherent 

 in X-ray measurements, Some results which I at first 

 obtained IT seemed to indicate that there was a slight increase 

 in the ratio with an increase of hardness of the primary. 

 But owing to the heterogeneity of the rays from an ordinary 

 tube, and the difficulty, at that lime, of sorting out and 

 using rays of a single penetrating power, these results were 

 not very convincing. 



Since these results were obtained, however, the work of 

 Prof. Barkla and others on " Fluorescent " X-radiations, a 



* Communicated by the Author. 



t Trans. Boy. Soc. South Australia, vol. xxxii. May 1908. 



X Bragg & Glasson, he. cit. Oct. 1908. 



§ Cooksey, 'Nature,' vol. Lxxvii. p. 509 (1908). 



II Stublmann, Phil. Mag. vol. xxii. p. 854 (1911). 



H 'Nature/ vol. lxxxii. p. 128 (1909). 



