PLEOCHROIC HALOES 143 



Haloes which are uniformly dark all over as 

 described above are, in point of fact, " over-ex- 

 posed " ; to borrow a familiar photographic term. 

 Haloes are found which show much very beauti- 

 ful internal detail. Too vigorous action obscures 

 this detail just as detail is lost in an over-exposed 

 photograph. We may again have " under- 

 exposed " haloes in which the action of the several 

 rays is incomplete or in which the action of certain 

 of the rays has left little if any trace. Beginning 

 at the most under-exposed haloes we find circular 

 dark marks having the radius 0*012 or 0*013 mm - 

 These haloes are due to uranium although their 

 inner darkening is doubtless aided by the passage 

 of rays which were too few to extend the darken- 

 ing beyond the vigorous effects of the two 

 uranium rays. Then we find haloes carried out 

 to the radii 0*0 1 6, 0*018 and 0*019 mm. The last 

 sometimes show very beautiful outer rings having 

 radial dimensions such as would be produced by 

 radium A and radium C. Finally we may have 

 haloes in which interior detail is lost so far out 

 as the radius due to emanation or radium A, while 

 outside this floats the ring due to radium C. Cer- 

 tain variations of these effects may occur, mark- 

 ing, apparently, different stages of exposure. 

 Figs. 6 and 7 illustrate some of these stages ; the 

 latter photograph being greatly enlarged to show 

 clearly the halo-sphere of radium A. 



In most of the cases referred to above the 

 structure evidently shows the existence of con- 



