ABSTRACT
J. Joly discusses that the pleochroic halos observed in mica are to be ascribed to the action on the mica of the α rays emitted by small inclusions of radio-active material. The radius of the halo, about 0. 04 mm. indicates that the effect is due in large part to the rays from radium. After the emanation had decayed, the inner wall of the capillary was seen under the microscope to be surrounded by an area of distinct colouration extending about 0. 04 millimeter from the walls. The outer boundary of the coloured region was sharply defined, and the depth of the colouration was equivalent to the maximum distance of the most penetrating a particle from the active matter. This result strongly confirms the correctness of the explanation of halos given by Joly.
