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ON A WANT OF SYMMETRY SHOWN BY SECONDARY X-RAYS. 



By W. H. Bragg, M.A., F.R.S., Elder Professor of Mathe- 

 matics and Physics in the University of Adelaide, and 

 J. L. Glasson. 



[Read October 6, 1908.] 



On the assumption that the Rontgen rays consist of 

 aether pulses it has been shown by J. J, Thomson ("Cond. of 

 Elect, through Gases," p. 323) that it is possible to account 

 for the existence of secondary Rontgen rays by as- 

 suming that the primary pulses set in motion elec- 

 trons over which they pass, and cause them to be- 

 come new centres of radiation. If the electron easily 

 follows the guiding force of the primary pulse, then 

 the secondary radiation resembles the primary in quality. 

 But if the electron is hampered by attachments to other 

 portions of the atom to which it belongs, then the new pulse 

 has not the same quality as the old ; the time of motion of 

 the electron is dragged out, and the pulse produced is softer. 



Now, if an electron becomes in this way a centre of 

 radiation the intensity of the secondary effect must be sym- 

 metrical about the line of motion of the electron. In par- 

 ticular, the intensity of the secondary radiation must be sym- 

 metrical about a plane passing through the electron perpen- 

 dicular to the primary ray, since this ray contains the line 

 of motion referred to. This deduction forms an integral 

 part of Thomson's theory of secondary Rontgen radiation, 

 and its truth has been assumed in calculations intended to 

 show that experimental results are in agreement with theory. 

 Barkla proves the same deduction in a paper published in 

 "The Philosophical Magazine" of February, 1908. 



Now it has recently been shown (Bragg and Madsen, 

 Trans. Roy. Soc, S.A.', May, 1908) that the cathode radi- 

 ations excited by y rays show a very marked want 

 of symmetry about the plane normal to the exciting ray ; 

 and again (Madsen, Trans. Roy. Soc, S.A,, July, 1908) 

 that there is a similar want of symmetry in respect to the 

 secondary y rays. The y rays and X-rays resemble one an- 

 other so closely in all their known properties that it is fairly 

 safe to assume any effect found to be true of the one kind 

 to be true also of the other kind, though perhaps to a dif- 

 ferent degree. In this case indeed Cooksey ("Nature," April 



