On the Photographic Action of a, ß and y Eays emitted from Radioactive Substances. ^ 



to emit, it was developed wlien a fine spot became visible to tlie 

 naked eye. On examining the plate under a microscope, the said 

 spot was found to consist of a multitude of separate trails of silver 

 grains running radially from a common centre. 



In the photographs taken in this way, however, there is 

 around the centre a dark area, which is no doubt the portion 

 where the active needle touched the plate during the above said 

 process. In this case, the radial tracks do not emerge at the 

 centre but at the rim of the dark area. Since the tip of the needle 

 had the sliape of a truncated cone terminating in a flat section of 

 about 10 //in diameter, it was impossil)le to obtain by this method 

 a photograph showing no central dark area. Still, this method 

 was found useful when, as reference will be made later, deflexions 

 of a ray tracks had to be investigated. 



Several trials were made to obtain photographs with nuclei 

 as small as possible. When an iron ball coated with the active 

 deposit was held above a photographic plate and knocked with 

 a hammer or the like, some fine dust particles adhering to the 

 iron ball seemed to I)e set free by the shock and to settle down on 

 the plate, becoming thus the sources of the « radiations. On 

 developing the plate several spots appeared and some of them 

 were found to possess nuclei whose linear dimensions are very 

 small compared with the length of the « ray track. The spots 

 themselves were so small that they were hardly detected by the 

 naked eye. 



To obtain a large number of these spots, it was found 

 effective to strike the photographic plate directly with the active 

 iron ball on the film side. To illustrate the general feature of 

 the photographs obtained in tliis way, one of the plates is 

 reproduced in fig. 1, enlarged 120 diameters. The irregular dark 

 areas seen in this figure are the spots where the plate was struck 

 by the iron ball. Around these areas, there can be seen several 

 circular spots, each of which consists of a set of a ]'ay tracks 

 ]'unning radially from a common centre. 



It may be remarked that, in a photograph in which a large 

 number of « ray trncks radiate from a common centre, each trail 



