﻿Phot ogvapl dc Action of a, ft, and 7 Rays, 839 



of contact of the active tip with the sensitive plate, or, when 

 the needle is held at a short distance above the plate, a point 

 immediately below it. When the period of exposure is 

 a sensible interval, a dark circular nucleus is formed. 

 A careful examination of the radiating tracks will show the 

 presence of an occasional track which is bent as the result 

 of scattering. The a particles appear to suffer only one 

 encounter, no tracks with more than one bend being visible. 

 The peripheral tracks appear to be longer than those nearer 

 the centre, but this is simply an optical effect, the central 

 tracks being foreshortened. Fig. 1 shows this clearly. 



It may be mentioned that, as was to be expected, the 

 tracks produced with thorium active deposit were longer 

 than those obtained with polonium. The longest track with 

 thorium active deposit measured 48*15//,, while the average 

 length of the ten longest seen near the periphery was 32*4 yu,. 

 The longest track observed consisted of only 15 silver grains, 

 while another track which measured 42*0 fju was made up of 

 24 silver grains. The average number of grains in the ten 

 longest tracks observed was 16*8. In all the above cases, 

 care was taken to make sure by an examination with an oil 

 immersion lens that the tracks did not consist of two 

 separate ones in continuation of one another. Michl gives 

 the length of the longest track with 9 grains as 32 fju. Be 

 obtained tracks with 13 grains, Baisch mentions bent tracks 

 with 15 grains, while Kinoshita and Ikeuti observed with 

 radium C tracks with as many as 16 grains. 



The action of ft particles.— In 1911 C. T. R. Wilson*, in 

 his well-known condensation experiments, made the tracks 

 of ft particles through moist air visible. The trails in this 

 case were indicated by droplets of water condensed on the 

 path of the ft particles, thus forming cloud-like streaks 

 which were immediately photographed. No one, however, 

 seems to have obtained a photographic impression of separate 

 ft particles or studied a ft photograph. A comparison 

 between the a. and ft photographs reveals certain points of 

 interest and practical importance. 



The three active sources used in the experiments with 

 a particles were also employed with ft particles, the 

 a particles being, in all cases, sifted off by wrapping the 

 sensitive plate in paper of suitable thickness. Figs. 4 and 5 

 are microphotographs with ft rays, the first with a thorium 

 active deposit needle, and the second with the radium E 

 needle. In both cases, the plate was wrapped up in black 

 paper, in which it is ordinarily packed, as well as in one 

 * C. T. R. Wilson, Froc. Roy. Soc. 



