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XXXII. Pleochroic Halos. By J. Joly, F.R.S.* 



WHEN recently examining a section of a greisen from 

 Altenburg, Saxony (supplied to me by Krantz of 

 Bonn) I noticed that some of tbe pleochroic halos scattered 

 through the Muscovite presented an appearance which, so 

 far as I am aware, has not hitherto been referred to by 

 penologists. It might, indeed, escape the notice of any one 

 unaware of its significance from the point of view of the 

 theory which- ascribes to these halos a radioactive origin. 

 I may add that I have several times sought for the structural 

 feature now to be described, but hitherto in vain. 



The appearance referred to is that of an inner and an 

 outer halo of very different densities; so that an inner darker 

 sphere and an outer less deeply coloured shell are clearly 

 indicated. In the field the central dark area is fairly uniform 

 save along the outer margin, where it diffuses rapidly, being 

 immediately succeeded by an area, fading somewhat — almost 

 imperceptibly — outwards, but wonderfully sharply denned at 

 the extreme boundary. The radial width of this corona is 

 about one half the radius of the inner disk. It is considerably 

 fainter in colour. Its diameter is easily measured by a mi- 

 crometer eyepiece. The diameter of the inner sphere is not 

 so well defined, but by careful focussing very consistent 

 measurements are obtainable. There is no doubt as to the 

 closely approximate truth of these measurements. Trials 

 were made to see by how much the travelling line of the 

 micrometer could be displaced on the margin without sensible 

 error ; it was found that a very few scale-divisions limited 

 the permissible displacements. 



There is unfortunately great difficulty in reproducing 

 photographically the appearance presented. This is because 

 the halo is imbedded in a medium far from flawless. When 

 under observation it is easy to focus through these flaw< ; 

 but in the photographs they bring in lights which confuse 

 the margins. 



The following are the readings taken on three such halos. 

 The readings are taken in different azimuths, which are 

 defined by colons ; and are the diametrical lengths in small 

 divisions of the micrometer head. 



A. Halo seen on a cleavage section. 

 Corona 84, 84, 83 : 83, 85, 85. Mean 84. 

 Halo 51, 52 : 53, 54, 52. Mean 52*4. 



* Communicated by the Author. 



