III. The genesis of pleochroic haloes

Abstract

In an earlier paper on the subject of pleochroic haloes it was pointed out that several features of the halo derived from the uranium family of radioactive elements were not easily accounted for. This fact led me to undertake a more careful study of haloes. But, although many observations and measurements were made, until recently my attempts at obtaining solution of the difficulties were too crude to merit publication. They were more especially hampered by the scarcity of haloes derived from the thorium family of elements. I could find out nothing as to the mode of origin of these haloes, and, on the observations available, it even appeared as if there were some unaccountable difference in the course of development of the haloes derived from the uranium and the thorium families. The initial stages of development seemed to be entirely absent in the case of the thorium series. This was not satisfactory. A few months ago, however, I found in a Vosges granite, a mica which was rich in thorium haloes. Some of these haloes appeared in the earlier stages of development; stages corresponding to the earliest in the development of the uranium halo. This find showed that the same course was followed in the genesis of both kinds of halo. The find was also important in another respect. Certain small discrepancies between the observed and the theoretic dimensions of the uranium halo had been forced on me as the measurements became more refined. This, of course, led me to distrust the basis upon which I was going in attempting to define theoretic dimensions. But in the developing thorium halo it would appear as if the basis of my predictions was uniformly supported. This seems to show that there is something anomalous, according to our existing knowledge of the ranges, in the early development of the uranium halo.