PLEOCHROIC HALOES 147 



examination of the outer edge of the circles always 

 reveals a steep but not abrupt cessation of the 

 action of the ray. Now Geiger has investigated 

 and proved the existence of scattering of the 

 alpha ray by solids. We may, therefore, suppose 

 with much probability that there is the same scat- 

 tering within the mineral near the end of the 

 range. The heavy iron atom of the biotite is, 

 doubtless, chiefly responsible for this in biotite 

 haloes. I may observe that this shading of the 

 outer bounding surface of the sphere of action is 

 found however minute the central nucleus. In 

 the case of a nucleus of considerable size another 

 effect comes in which tends to produce an 

 enhanced shading. This will result if rays pro- 

 ceed from different depths in the nucleus. If the 

 nucleus was of the same density and atomic 

 weight as the surrounding mica, there would be 

 little effect. But its density and molecular weight 

 are generally greater, hence the retardation is 

 greater, and rays proceeding from deep in the 

 nucleus experience more retardation than those 

 which proceed from points near to the surface. 

 The distances reached by the rays in the mica 

 will vary accordingly, and so there will be a 

 gradual cessation of the effects of the rays. 



The result of our study of the halo may be 

 summed up in the statement that in nearly every 

 particular we have the phenomena which have 

 been measured and observed in the gas repro- 

 duced on a minute scale in the halo. Initial re- 



