62S Mr. W. F. Rawlinson on the Decrease 



in 



passing through matter. Yon Baeyer* also attacked the 

 problem by a photographic method, and obtained results 

 similar to those of Wilson. 



The first definite quantitative results were obtained by 

 Danysz by the photographic method. He used an a-ray 

 tube as a source of /3-rays, and examined the radii of cur- 

 vature of the paths traced out by the /3-rays in a uniform 

 magnetic field, both when the source was bare, and also when 

 it was covered with a thin layer of stopping material. Now 

 if H is the value of the magnetic field in gauss, and p the 

 radius of curvature of the path of the /3-particle, then Hp is 

 proportional to the velocity of the yS-ray. It is known that 

 the /3-rays from radium B and radium C contain several 

 groups of rays with perfectly definite velocity, and these 

 nroduce a series of circles on the photographic plate. When 

 the source is covered, the radii of these circles are of course 

 reduced, and the difference in the values of Hp in the two 

 cases is proportional to the decrease in velocity of the given 

 /3-rays. Danysz took a photograph of the paths, first with 

 the source uncovered, then with it covered, and deduced the 

 change in H/> from the difference in the radii of the circles 

 on the two plates. His results show no systematic variation^ 

 and he concludes that to a first approximation a sheet of 

 substance of a given thickness produces on particular /3-rays- 

 a stopping proportional to its densityf. The accuracy of his 

 results may, however, have been impaired by variations in 

 the magnetic field, and also by the fact that he measured the 

 small difference between two large values of H^o which were 

 obtained in separate experiments. 



Dr. Bohr has worked out a theory of the decrease of velocity 

 of electrified particles in passing through matter. He con- 

 cludes that I/3 3 should be approximately constant for any 

 given substance (I being the decrease in velocity in passing 

 through a screen whose weight per unit area is 0*01 grm., 



and £ the ratio velocity of 0-ray y According to him this 

 velocity of light / 



product should be less for elements with large atomic weights 



than for the lighter elements; also for any given substance 



it should increase slightly with increase of velocity of the 



/3-rays. The results of Danysz are not sufficiently accurate 



to afford a real test of the theory, and the following work 



was undertaken in the hope of obtaining further evidence on 



the subject. 



* v. Baeyer, Phys. Zeit. xiii. p. 485 (1912). 



t Dacysz, Comptes Rendus, July 1911 ; Le Radium, Jan. 1912. 



