148 NATURE OF THE RADIATIONS [CH. IV 
primary rays. It is thus not surprising that the secondary rays 
set up by the 8 and vy rays may in some cases produce a photo- 
graphic impression comparable with, if not greater than, the effect 
of the incident rays. 
On account of these secondary rays, radiographs produced by 
the 8 rays of radium in general show a diffuse border round the 
shadow of the object. For this reason radiographs of this kind 
lack the sharpness of outline of X ray photographs. 
101. Mme Curie’ has shown by the electric method that the 
a rays of polonium produce secondary rays. The method adopted 
was to compare the ionization current between two parallel plates, 
when two screens of different material, placed over the polonium; 
were interchanged. 



g ipsa | Thickness | Current | 
creens employe inmms. | observed | 
Aluminium BOE 0-01 | 17-9 
Cardboard a 0:005 | ‘ 
| Cardboard oe 0-005 6-7 
| Aluminium zeae |e 10,011 
| Aluminium aay OnTO201 150 
pein oe nO: 005 ; 
Din 25 lO ;005 ve 
Aluminium sels y OO ee 
Tin idee iter! 0-005 i 
Cardboard a) 07005 139 
Cardboard ARS 0-005 4-4 
Tin ae Sa 0-005 

These results show that the a rays of polonium are modified in 
passing through matter, and that the amount of secondary rays set 
up varies with screens of different material. Mme Curie, using the 
same method, was unable to observe any such effect for the 8 rays 
of radium. The production of secondary rays by the @ rays of 
radium is, however, readily shown by the photographic method. 
1 Thése présentée a la Faculté des Sciences, Paris 1903, p. 85. 
