Pleochroic Halos

Abstract

IN the March number of the Philosophical Magazine I gave reasons for believing that the pleochroic halos of certain micas are referable to the radio-activity of the enclosures which are invariably associated with the halos. I cited in favour of this view the facts that all minerals definitely recognisable as forming the centres of halos are radio-active; that the perfect sphericity of the halo in a medium such as mica precluded any explanation involving diffusion or segregation of colouring matter, and that there was a very exact agreement between the known effective penetrability of rays in media of this density and the maximum radius attained by the halo. I subsequently found; by examination of specimens kindly lent to me by Dr. Teall, that the same reasoning was applicable to halos formed in cordierite.

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