ABSTRACT
The study of pleochroic haloes essentially came to an end with the outbreak of the Second World War. Its ending as a subject of research, however, was also marked by the appearance in the Proceedings of the Royal Society of London of two interesting papers by G H Henderson. The remarkable feature that Henderson and Sparks noticed about these circles was that their radii corresponded to the different ranges of the alpha particles emitted by three isotopes of polonium occurring in the uranium-238 chain. The only potential difficulty with this explanation was that many type-C haloes did not seem to be lying along obvious cracks in the micas. So Henderson said that this must mean that fluids can penetrate uncracked micas. The mechanism would be an exact replica of that proposed by Henderson for the point precipitation of polonium in micas.
