﻿Intensity of Radiation from Radioactive Sources. 157 



just outside the point R, due to the side AB, is nearly equal 

 to that due to the side AD. Consequently, the intensity o£ 

 the radiation to the left of the point R is about twice that to 

 the right of R in the region RS. 



This conclusion was confirmed by a slight alteration of the 

 experiment. Two thin plates of metal, which completely 

 absorbed the a rays, were placed in the positions AR and 

 DS. The radiation reaching a point in the region RP is 

 then due entirely to the rays from the side AB. Under such 

 conditions, it was observed that the intensity of the photo- 

 graphic impression in the region RS was very nearly the 

 same as that in the region RP close to R. 



Some very beautiful optical effects can be produced by a 

 different arrangement of the experiment. The active rod 

 rests on the phosphorescent screen with its axis normal to the 

 surface. The illuminated area is seen to be divided by four 

 dark bands at right angles to each other, forming a well- 

 marked cross. The width of these bands is the same through- 

 out their length and equal to the side of the square. The 

 bands correspond to the regions included in fig. 3 between 

 the lines AA T and DD l5 AA 2 and BB 2 , &c\ The effects ob- 

 served are best illustrated by means of the reproduction of 

 an actual photograph shown in fig. I, PL II. The active 

 rod rested normally on the photographic plate, as in the case 

 of the experiment with the screen. 



The explanation of these bands, where the luminosity is 

 less than in the region immediately outside them, is obvious 

 from what has already been stated. 



In most of the experiments, the rod was charged negatively, 

 then exposed in the emanation vessel, in order to concentrate 

 the active deposit upon it. The active deposit was conse- 

 quently not uniformly distributed, being in somewhat greater 

 amount at the edges of the rod. This lack of uniformity in 

 the distribution had. however, no sensible effect on the main 

 features of the photographs, for equally clear effects were 

 obtained when no electric field was acting during the ex- 

 posure of the rod to the emanation. "With an active rod 

 clear photographs of this character can be obtained in a few 

 minutes. 



It was thought of interest to examine the distribution of 

 the radiation in two other cases. In one experiment, the 

 cross section of the rod was an equilateral triangle of 6*5 mms. 

 side, and in the other a regular hexagon of 2' 75 mms. side. 

 The rods were placed normally on the plate, and the photo- 

 graphs obtained are shown in figs. 5 and 6, PI. II. 



The explanation of the results obtained is clearly seen from 



