578 Dr. A. B. Wood on Long-range 



supplied in a paper by Marsden and Bichardson *. The 

 values thus deduced were afterwards carefully checked by 

 direct observation. When mica was used as absorber of the 

 8'6 cm. a-particles, two thin layers, each of air-equivalent 

 4 3 cm., were superposed, the total " effective " air-equi- 

 valent from active source to ZnS screen in that case being 

 8*6 cm. + -"45 cm. or 9*05 cm. Thus any particle originating 

 in the active material and striking the zinc-sulphide screen 

 must have a range greater than 9 cm. In certain experi- 

 ments aluminium screens were used to stop the 8* 6 cm. 

 a-particles. For this purpose four layers, each of 2*0 cm. 

 air-equivalent, and one layer of 0*5 cm. air-equivalent, were 

 superposed, the total effective air- value from active source 

 to ZnS screen being in this case 8'95 cm. 



The methods of counting scintillations and experimental 

 precautions necessary have frequently been described else- 

 where, and need no further mention here. 



§ 3. Relative number of Long-range a-particles and Oxygen 

 atoms. 

 In order to discriminate between the scintillations produced 

 by a-particles and oxygen atoms, two methods have been 

 employed : — 



(a) By varying the position of a mica absorbing screen 

 between the active source and the zinc-sulphide 

 screen. 



(l>) By comparing effects with mica and aluminium 

 absorbing screens. 



(a) In this method the distance between the active source 

 and the zinc-sulphide screen is kept constant, whilst the 

 mica absorbing screen (of air-equivalent 8*H cm.) is placed 

 (1) near to the active source, (2) near to the zinc-sulphide 

 screen, the number of "long-range" scintillations being 

 carefully determinedfor each position of the mica (see fig. 2). 

 In this comparison the brass aperture (fig. 1) restricting 

 the divergence of the X-ray beam incident on the mica is 

 removed. It will readily be seen that if all the long-range 

 particles come directly from the active deposit, no appre- 

 ciable difference in the number of scintillations counted 

 should be observed as the mica is moved from position (1) 

 to (2). If, however, the long-range particles all originate 

 in the mica, it can be shown that the number of scintilla- 

 tions observed in case (1) when the mica is close to the 



* Marsden & Richardson, Phil. Mag. Jan. 1913. 



