﻿422 Prof. S. Kinoshita and Mr. H. Ikeuti on the 



are radiating from a centre, each trail does not appear 

 isolated from the others along the whole length, but, in the 

 vicinity of the centre, comes very close to or overlaps the 

 neighbouring ones. This effect gives rise to another circular 

 dark area in the middle, though it has no well-defined 

 boundary. The radius of the dark area varies of course as 

 the number of the trails. As this dark area covers the dark 

 nucleus above spoken of, it sometimes makes the boundary 

 of the latter indistinct. 



The radius of the halo varies slightly in different plates 

 from one another, but the length of each constituent radial 

 trail or the difference of the radii of a halo and the circular 

 nucleus inside it, is the same for all the haloes and equal to 

 about *054 mm. in the case of Ilford's Process Plates. It is 

 most probable that this represents the range of the a particles 

 from radium G in the substance, as, in these cases, we used 

 active deposit in which radium A had practically decayed 

 away. The silver grains constituting this set of trails are not 

 restricted within the uppermost layer of the film, but found 

 also in layers within some depth beneath the surface. From 

 these facts, it is evident that the halo is produced by a 

 similar process as the pleochroic halo seen in the mineral 

 such as biotite, and investigated in detail by Joly. In 

 the present case, however, a spherical halo cannot be 

 expected, as the film on the plate employed was equivalent 

 to only about 2 cm. of air in stopping the « rays, as 

 calculated from its weight '0030 gramme per square centi- 

 metre *. In order to show the haloes at different stages 

 of formation, we have reproduced some of the micro- 

 photographs in figs. 1-6 (PL VII.). These were taken 

 with a microphotographic apparatus by Zeiss, the magnifi- 

 cation being 380 diameters. Fig. 1 shows a halo in which 

 about 80 tracks are to be seen, this being of course the 

 number of tracks on the plane focussed in reproducing and 

 forming only a small fraction of the total. The tracks which 

 do not radiate from the centre are due to other sources lying 

 outside the halo. A fairly developed halo is shown in fig. 6, 

 which, on our estimation, contains about 200 tracks. The 

 spots covering the greater part of the figure are due to dirt 

 on the plate. Figs. 2, 3, 4 and 5 are haloes at intermediate 

 stages of formation. On the plate containing the halo in 

 fig. 2, there are, besides, several extended sources of radiation,, 

 whose effect is visible on one side of this figure. We have 

 counted the number of grains in each trail and found it to be 

 about 16 on the average. 



* Kinoshita, /. c. p. 487. 



