328 Prof. J. Joly on Pleochroic ITalos. 



B. Halo on a basal section. 

 Corona 83, 81, 80 : 84, 88 : S5, 88 : 84, 8G, 84. Mean 83-3. 

 Halo 50, 54, 52 : 54,56,56 : 59,56.55 : 59,55,55. Mean 55*2. 



0. Halo on an approximately basal section. 

 Corona 80, 81 : 82, 81 : 80, 80, 79. Mean 80*4. 



Halo 52, 49 : 50, 48, 51 : 49, 51, 52. Mean 50*3. 



That no unconscious bias affected these readings will be 

 evident when I add that the whole of them were obtained 

 before the calculations given below were made. 



With the same optical arrangements it was found as the 

 mean of several readings that an engraved one-tenth of a 

 millimetre was traversed by 122 small divisions of the head. 

 Accordingly, we have in millimetres for the radial measure- 

 ments of the halos and their coronas :— 



Corona. Halo. Calc. radius of Halo. 



A 0-084 0-022 0-023 



B 0034 0-028 0-023 



C 0-033 0-021 0-022 



The inference that the complex structure of these halos 

 finds an explanation in the varying ionization ranges of the 

 a rays of the uranium-radium family is based on the following 

 considerations. 



The effective ranges of these rays, as measured by Ruther- 

 ford and later by others, will account for the measurements 

 made above, The most accurate determinations of the ranges 

 are probably those of Bragg (Phil. Mag. Sept. 1905). Bragg' s 

 results are as follows : — 



Radium 3*50 cms. 



Emanation or Radium A. 4*28) cms. 



Radium A or Emanation. 4*83 cms. 



Radium C 7*06 cms. 



To these measurements may be added those of Levin (PJn/s. 

 Zeit. 1906, p. 461) for RaF, 3*85 cms. and by Allen (Phys. 

 Rev. 1908) for uranium 8*5 cms. 



It is apparent that a particle of radium in equilibrium will, 

 in air, be surrounded by a sphere of very intense ionization 

 which will be limited by a radius of 4*83 cms., and beyond 

 this for a further distance of 2'23 cms. there will be a shell 

 within which the ionization is comparatively feeble, pro- 

 bably only of one-sixth or one-seventh the intensity of that 

 prevailing within the inner sphere, and of course still further 

 diminished by the divergence of the rays. 



If we calculate from the extreme radial dimensions of the 



