6 Art. 10. — S. Kinoshita and H. Ikeuti : 



in air. According to Marsden and Ricliardson'', the above ratio 

 for silver is much higher, amounting to *S6. In the present case, 

 the absorption of the a rays is mainly due to gelatine, which is the 

 main composition of the sensitive fihn, and this substance seems 

 to be of a character similar to air concerning absorption. 



6. Number of silver grains along an a rag track. Silver grains 

 along an « ray track are naturally not equidistant. To find the 

 distribution, Ikeuti has measured, by means of the ocular micro- 

 meter of a micro.scope, the distance d between successive grains on 

 the track for a large number of pairs of the grains. When the 

 number of pairs of grains was plotted against the corresponding 

 values of d, and a mean curve was drawn, this showed a maximum 

 at d=l'Sö fj. in the case of Ilford Process Plates, falling quickly to 

 zero on the side of the origin and somewhat slowly on the other side. 

 From tliese measurements the mean value of d was calculated to be 

 !2'85 /ji. This corresponds to 350 grains per millimetre of the track. 

 Since the longest tracks of the «rays from radium A and radium C 

 are 36 and 52 // respectively, they will consist on the average of 12'6 

 and 18'2 grains respectively. It must, how^ever, be remembered 

 that the actual numbers are subjected to faii'ly large fluctuations. 



7. The nature of the 'photographic action of a particles. Alpha 

 ray tracks obtained in a sensitive film do not exhibit even the 

 slightest difference along their wliole range; neither in the com- 

 pactness of the grains nor in their size. This is a very important 

 fact and confirms conclusively the previous experiment of Kino- 

 shita"\ in which the constancy of the ])hotographic action of an 

 a particle along the whole range was photometrically established. 

 Now, if it be considered that there are all possibilities of regarding 

 ionisations in a solid and in a gas to be of a similar type, the 

 question may arise, why the photographic action, which is 

 nothing but the result of ionisation, should not be represented by 

 the characteristic curve known as Bragg' s ionisation curve. As 

 a matter of fact, darkening in the pleochroic halo is particularly 

 pronounced near the boundary, in spite of that the number of a 



1) E. Marsden and H. Eichardson, Phil. Mag. 25 (1918), p. 191. 



2) I. c. 



