a Particles from Thorium Active Deposit. 707 



been observed, the deviations of the measured velocities from 

 those calculated from Geiger's relation falling well within 

 the limits of error involved in the measurement of the ranges 

 of the a-particles, and hence within the limits of error in the 

 calculation of the velocities. 



Copies of original photographs are shown in figs. 3 and 4, 

 PL XVII. In fig. 3 the originals are reproduced actual size 

 (by direct contact printing): fig. 4 is an enlargement of (a) 

 and (c) in fig. 3. Photographs (a), (6), and (c), fig. 3, were 

 obtained using approximately the same magnetic field in 

 each case but different distances (/) from source to slit. 

 Thus in (a) l='d'() cm., (b) Z = 3*5 cm., (c) Z = 4*0 cm. It will 

 be seen that, although the intensity of the lines decreases with 

 increasing distance, the sharpness remains good in all cases. 



An examination of these photographs reveals another in- 

 teresting feature. In addition to the sharpness of the lines, 

 their relative intensity is quite noticeable. Rough observa- 

 tions of the two deflected lines, on either side of the central 

 line, show that the intensity of the least deflected line (due 

 to the swift a-particles of S'O cm. range) is at least double 

 the intensity of the more deflected line (due to slower 

 a-particles of 4*8 cm. range). Many observers* have shown, 

 directly and indirectly, that the number of particles emitted 

 from thorium active deposit with a range of S'G cm, is 

 approximately double the number emitted with a range of 

 4*8 cm. Thus Marsden and Barnittf showed this to be the 

 case by both ionization and scintillation methods. The 

 results given in the present paper are interesting as being 

 the first attempt to illustrate this point by a photographic 

 method. With regard to this question, however, no final 

 statement can be made until definite information is obtained 

 as to the relative photographic effect of a-particles of dif- 

 ferent penetrating powers. Thus it seems probable that an 

 a-particle of range 8*6 cm. will produce a greater blackening 

 of the film than an a-particle of range 4*8 cm. This question 

 is at present under investigation. 



2. Direct comparison of the velocities of the a-particles from 

 thorium active deposit with, the velocity of the a-particles 

 from radium C. 



At the suggestion of Prof. Sir E. Rutherford a number of 

 experiments have been made to obtain an absolute value for 

 the velocities of the a-particles, using the standard value 



* e. q. Bronson, Phil. Mag. xvi. p. 291 (1908). C4eiger and Marsden, 

 Phys. Zeit. xi. Jan. 1, 1910. 



t Proc. Phys. Soc. pp. 50-61, Dec. 1911. 



