An Experimental investigation of the Nature 

 OF THE 7 Rays. No. 1. 



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

 matics and Physics in the University of Adelaide, and 

 J. P. V. Madsen, D.Sc, Lecturer on Electrical En- 

 gineering. 



[Received January 2, 1908 : read May 5, 1908.] 



In papers recently published in the "Proceedings of the 

 Royal Society of South Australia" (May and June, 1907) 

 and in the "Philosophical Magazine" (October, 1907) an at- 

 tempt was made to show that the ether-pulse theory of 7 and 

 X-rays might prove to be incorrect after all, and that most 

 of the known properties of these rays could be explained 

 more simply and directly on the supposition that they were 

 material and consisted of neutral pairs. The arguments 

 were based on comparison of known phenomena with deduc- 

 tions from each of the two opposing hypotheses. At that 

 time there did not seem to be any opportunity of appeal to 

 a decisive experiment. 



The object of this paj^er is to give a preliminary account 

 of an investigation which appears to us to give a final answer 

 as regards the 7 rays, and to show that they are material in 

 nature. 



Secondary radiation, which is excited in an atom by 

 a passing wave or pulse, must be distributed symmetrically 

 with regard to a plane passing through the atom perpendicu- 

 lar to the direction of motion of the pulse. If we speak of 

 the primary pulse as going forwards, the secondary radia- 

 tion is just as likely to go backwards as forwards. This is 

 a well-recognized principle. For example, J. J. Thomson 

 divides the secondary radiation, due to 7 rays, into two equal 

 parts, which he supposes to move away symmetrically in 

 opposite directions, and, for convenience of calculation, paral- 

 lel to the direction of the primary rays (Cond. of Electricity 

 through Gases, p. 406). Supposing, therefore, a pencil of 

 y rays to pass normally through a plate so thin that its 

 absorption may be neglected, the secondary radiation should 

 be exactly the same on the two sides of the plate — in amount, 

 in quality, and in distribution ; and it ought not to be pos- 

 sible to discover, by any comparison of the secondary radia- 

 tions on the two sides, which is the face of entry and which 

 of emergence. 



