Pleochroic Halos. 645 



permanently overcome. The true explanation may, on the 

 other hand, lie in effects not yet suspected. 



The further development of halos is marked by widening 

 of the corona and advance outwards of the pupil. In general 

 there seems to be a pause after the effects of RaA have brought 

 the pupil out to such dimensions as are given in the foregoing 

 table. In many old halos, as described in the first part of 

 this paper, the differentiation is limited to the pupil and 

 corona— the latter a more lightly shaded continuation of the 

 former. But for some reason the development does not 

 always progress as if leading to this result. Thus in the 

 mica we are dealing with we have found halos with a fairly 

 uniform dark pupil brought out to a radial distance of 0*0278, 

 and still separate from the corona. That this is not due to a 

 large central nucleus is shown by the outside radius of the 

 corona — 0*0335 mm. — and can perhaps best be explained on 

 the activity of several nearly coincident actinium rays ranging 

 over 5 cm. in air. If this is the normal process of develop- 

 ment — that is by the gradual advance of the pupil — the state 

 of simple differentiation into pupil and iris must be explained 

 on the subsequent greater accumulation of effects within the 

 limits reached by RaA. But it must be admitted that while 

 the general features of the development seem free from any 

 obscurity, the relative intensity and order of the events are 

 not wholly explicable. We are, however, in total ignorance 

 of what really takes place in the crystalline medium. 



The highest magnification applied to halos does not appear 

 to throw light upon the latter point. A faintly radiate 

 structure is sometimes seen, in particular lighting, in halos 

 which are under-exposed. This may sometimes appear to 

 be confined to the corona. Its significance, if it has any, is 

 unknown. Doubtless helium is actually stored in the suc- 

 cessive shells which mark the limits of each ray. Under 

 what conditions, whether free or possibly in synthetic com- 

 bination with atoms into which it has penetrated, is not 

 known. Strutt's results on the helium stored in various rocks 

 and minerals point to a mere mechanical storage of the helium. 

 It seems very improbable that the presence of the helium 

 occasions the darkening observed. There is really very little 

 of it. As will be seen later there is, probably, not sufficient 

 to create an atmosphere pressure if distributed throughout 

 the halo. Again, if the helium is the cause of the darkening, 

 why are halos confined to certain minerals ? The restrictions 

 are not due to want of radioactive nuclei, for zircon and 

 apatite, which in some minerals give rise to halos, in others 

 in the same rock-section are without effect; and, as observed. 



