﻿7 racks of the a. Particles in Sensitive Films. 421 



naked eye. As a matter of fact, it was not essential to keep 

 the needle in contact with the plate throughout the time of 

 exposure. A momentary touch seemed to be sufficient, if a 

 proper interval of time was afterwards allowed to elapse 

 before development, indicating that a part of the active deposit 

 was detached from the needle and left behind on the plate at 

 the point of contact. In some cases, a metal piece coated 

 with the active deposit was, while being held above the plate, 

 knocked with a finger, when similar results were obtained. 

 In these cases, some dust particles adhering to the metal 

 piece must have been set free by the shock and have settled 

 down on the plate. 



On examining the plate under a microscope, the said spot 

 is seen to consist of a multitude of the radial trails of silver 

 grains around a circular dark nucleus, to which reference 

 will soon be made. A closer examination sbows that these 

 trails of grains are, in general, to be divided into two sets. 

 Those constituting the first set emerge at the rim of the 

 nucleus and end very nearly at the circumference of a circle 

 drawn outside the nucleus and thus present themselves as a 

 halo. The second set of the trails, on the other hand, spread 

 out around the nucleus over a wide region with no sharply 

 defined boundary. By focussing the microscope, it can be 

 ascertained that the silver grains constituting the latter set 

 of trails are all found in the uppermost layer of the sensitive 

 film. 



It was, however, not always the case that both of these 

 sets are equally conspicuous. In some cases, one of them 

 was particularly pronounced while the other was hardly 

 visible. 



The nucleus above mentioned is undoubtedly the cavity in 

 the gelatin film produced by the point of the needle when it 

 was brought into contact, and has nothing to do with the 

 a particles. Its size is various in different photographs, 

 depending on the circumstances under which the needle was 

 held ; the greater the pressure applied to the needle, the 

 larger the size of the nucleus. It may be added here that 

 the pointed end of the needle had the shape of a truncated 

 cone terminating in a flat section, which, as measured under 

 a microscope, had a diameter of about "01 mm. When the 

 needle had been held with such a care that it hardly touched 

 the plate, the photograph showed no dark nucleus. The 

 same fact was often experienced when an active dust particle 

 was the source of the u rays. Figs. 2 and 7 (PI. \\\.) 

 are the examples. 



In the photographs, in which a large number of trails 



