Pleochroic Halos


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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Author information

Rights and permissions

Reprints and Permissions

About this article


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.