Question of the Homogeneity of y- Rays. 733 



whereas in aluminium it is not used up so fast and it produces 

 less secondary radiation. 



But such'a soft /3-radiation will always be in equilibrium 

 with the soft 7-radiation, except over the part of the curve 

 not investigate owing to primary /3-rays. The logarithmic 

 curves obtained by subtracting the effect due to the hard 

 type from the observed effects should be straight for 

 aluminium no less than for lead, whereas it is obvious that 

 they are not. All that the explanation, if we understand it 

 aright, would seem to involve is that the slopes of the curves, 

 plotted against equivalent thickness, should be greater for 

 dense than for light materials. 



Part II. — Absorption of y Rays in truncated 

 Hemispheres. 



H. W. Schmidt (Ann. Phys. 1907, xxiii. p. 689) has given 

 the solution of the problem of the absorption of a homo- 

 geneous radiation from a point source uniformly distributed 

 in all directions, in a plate of uniform thickness placed 

 directly over the source, assuming that the absorption 

 proceeds exponentially as with light. His paper and the 

 theoretical absorption curve he gives suggested at first sight 

 that the departure of the experimental curves from the 

 exponential form might be due, not to any want of homo- 

 geneity of the rays, but to the obliquity of part of the beam. 

 This, however, as will be shown, is not the case. The error 

 so introduced is trifling with the disposition employed, the 

 found absorption coefficient being about 25 per cent. greater 

 throughout the whole range than the true theoretical coeffi- 

 cient for a parallel beam of rays. Schmidt solved the 

 problem in connexion with his study of /3-rays, to which 

 it will be shown the solution does not apply owing to 

 scattering. Experiments, however, on the absorption of 

 the y-rays with some new dispositions for which the absorp- 

 tion according to the theoretical expressions can be calculated, 

 have strongly confirmed the view that the 7. rays of radium 

 are homogeneous and that a soft primary type does not exist, 

 although still much remains to be cleared up. 



Unfortunately, in Schmidt's paper the accidental omission 

 of a factor in the final expression makes the solution appear 

 false, and we are indebted to Sir Joseph Larmor for solving 

 the problem for us independently. A plate of infinite area 

 and thickness t is in contact with a point source sending out 

 radiation uniformly in all directions which are absorbed 



