On the Photographie Action of a, ß and y Eays emitted from Radioactive Substances. 5 



4. Haloes due to radium C. Tliese can be obtained Ijy uti- 

 lizing as the source of « rays the active deposit of ra(hum, in which 

 radium A has ah'eady disappeared. Figs. 2-4 are the micro- 

 photographs of these haloes, enlarged 50U diameters. It will be of 

 interest to see the different stages of formation. In the halo in 

 hg. 2 about 4'") tracks are to ]>e seen, this being of course the num- 

 l)er of the tracks on a plane focussed in reproducing the micro- 

 photograph and forming therefore only a small fraction of the 

 total. The haloes in figs. 3 and 4 are seen to contain about 70 and 

 1 20 tracks respectivel3\ 



The ends of the tracks constituting a halo do not lie strictly 

 on a circle owing to the difference in the struggling of the rays 

 through the medium. \A'e have tlierefore taken as the radius of the 

 halo the radius of the circle drawn so as to pass through the ends of 

 most of the far reaching tracks. The radii of the haloes determined 

 in this way vary slightly, but their smallest limit is found to be 

 o2 [JL. Since, on the other hand, isolated tracks of this length 

 are found in such pliotugraphs as fig. 1), this may be taken as the 

 range of til e « rays from radium C in the substance, so that the 

 radiant nuclei of these haloes must l:)e ver}- small. 



5. Haloes due to radium A and radium C. To obtain haloes 

 of this kind, we have exposed an iron ball to radium emanation for 

 a few minutes and ])erformed the above stated process as quickly 

 as possible. These haloes are reproduced in figs. 5-8, in the same 

 magnification as before. They indicate that the tracks of a set of 

 homogeneous a rays from radium A give rise to another concentric 

 circle inside that due to radium C, as in the case of the pleochroic 

 halo. 



It will l)e seen that in fig. 5 the outer circle is more conspicu- 

 ous than the inner one, wliile with fig. (i the reverse is the case. 

 Which of the circles comes out more conspicuously depends upon 

 in what proportion radium A and radium C have been mixed in 

 the source utilized. 



Tlie inner circle is in each case smaller than the outer by K) // 

 in radius. Thus tlie ratio of the ranges in the sul)stance of the 

 two a rays becomes(52-l()) : 52 or 1)9, which is the same as its value 



