Prof. J. Joly on Pleochrtic Halos. 329 



corona the radius of the inner darker region, according to 

 the ranges given by Bragg for Ra C and Ra A or emanation, 

 we find for the inner halo the dimensions given in the third 

 column. The value of this test is of course independent of 

 the absorption or stopping-power of the muscovite, the 

 figures being simply comparative. It would seem probable, 

 from the agreement of the figures, that the corona is 

 accounted for by the very considerable difference between 

 the ranges of a rays from Iia C and from the other members 

 of the radium family. 



We may also estimate the theoretic radii of the outer and 

 inner halos by applying Bragg's law, that the ranges are 

 proportional to the square roots of the atomic weights. 

 Taking the formula of muscovite as 2H 2 0, K 2 0, 3A1 2 3 , 

 6Si0 2 , I find 4*2 is the average square root of the atomic 

 weight. For air the corresponding value is 3'79. From 

 these data, and assuming the density of muscovite to be 

 2*8, we get for the rays of 7*0f> cms. in air, a range in 

 muscovite of 0*0337 mm., and for rays of 4*83 cms. in air a 

 range in muscovite of 0*0230 mm. It will be seen that the 

 agreement is very exact. It would be even closer if a 

 slightly higher density was assumed for muscovite ; which 

 would he quite permissible. 



Measurements made in other rock-sections have given me 

 higher values for the limiting radial dimensions of halo-. 

 The possibility that thorium may he responsible for the 

 halo, in some cases, must be borne in mind. I have bad 

 cordierite exposed under specks of thorianite now for nearly 

 four months, as yet without definite coloration, hut taking 

 into account the feebleness of the radiation compared with 

 what can he commanded in similar experiments with radium, 

 this result is not surprising. I find that the a. rays of ThC, 

 which have been found by Hahn (Pliys. Zeit. 1906, p. 401) 

 to possess a range in air of <S-(; cms., would, on Bragg's law, 

 account for a halo in muscovite ofO'042 mm. radius. 



It may be asked why all pleochroic halos duo to radium 

 do not show the differentiated structure described above. 

 The answer is, I think, that in very old halos, or in halos 

 due to very active nuclei, this structure becomes obliterated ; 

 for somewhat the same reason as an over-exposed photo- 

 graphic plate loses contrast and detail. The effects of the 

 more intense inner ionization probably reach a maximum, or 

 at least do not continue to visibly increase at the initial rate. 

 Finally, the effects in the corona must accumulate till there 

 is sensible equality in density. In the greisen in which the 

 complex halos occur there are also found halos in the final 



