85b* Prof. Bragg and Mr. Glasson on a Want of 



symmetrical about the line of motion of the electron. In 

 particular, the intensity of the secondary radiation must be 

 symmetrical about a plane passing through the electron 

 perpendicular 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 

 another 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 

 different degree. In this case, indeed, Cooksey (' Nature,' 

 April 2, 1908) has already shown that the secondary cathode 

 radiations excited by X-rays are not at all symmetrical about 

 the normal plane, the emergence rays being greater than 

 the incidence, as in the case of the y rays. 



It remained, therefore, to examine the secondary X-rays 

 excited by primary X-rays ; and the experiments described 

 in this paper were made with that object. We find that in 

 general want of symmetry does exist, that it is sometimes very 

 pronounced, and that is in keeping with expectation based on 

 Madsen's study of the secondary y rays. Hard y rays show 

 a very large difference between the quantities of emergence 

 and incidence radiation ; for soft y rays the difference is 

 smaller. Since X-rays are to be looked on as a very soft 

 form of y rays, the difference should be smaller still ; and 

 this is what we have found to be the case. 



The general form of the apparatus which we have used 

 is shown in fig. 1. Variations of the upper portion of it are 

 shown in figs. 2 and 3. A small pencil of X-rays passed 

 upwards through apertures in lead plates at A and B, and 

 then along the axis of the ionization-chamber and out into 

 the open. In our first experiments the upper part of the 

 apparatus was arranged as in fig. 3. The primary rays did 

 not pass through the effective part of the ionization-chamber, 

 being separated therefrom by the cylindrical screen SS, 

 which could be made of various thicknesses and various 



