630 Prof. J. Joly and Mr. A. L. Fletcher : 



the reflected sunlight are separated, there is not a large 

 portion of nnpolarized light in the latter, a serious objection 

 to a radiation pressure theory of the tails appears at once. 



When a comet has a dense bright nucleus, a certain small 

 proportion of its intrinsic light must show evidence of 

 polarization, due to scattering by the tail, of light emanating 

 from the nucleus. But this effect would probably be too 

 small to be observed in any case. Radiation from the nucleus 

 may also, in some cases, give a radiation pressure assisting 

 that of the sun. although the gravitational action of the 

 nucleus can never be important. 



All observations relating to polarization would be liable to 

 be affected by the presence of fluorescence, and in view of 

 the results obtained by Wood *, deductions from them demand 

 as much caution as those from the observed brightness. 



LXVJL Pleochrok Halos. By J. Jolv. F.R.S., and 

 Arnold L. Fletcher, B.A.I.^ 



[Plates YIII. & IX.] 



SINCE the date of an earlier paper on the subject of 

 Pleochroic Halos J the examination of many halos of 

 special interest has elicited points of sufficient importance to 

 justify fuller consideration oi the subject. 



In the paper referred to it was shown from the measure- 

 ment of: halos in a Greisen that the radial dimension of the 

 corona, or outer halo, when compared with that of the 

 darker central area was such as to support the conclusion 

 that the corona represents a shell of less complete ionization 

 for other effect) due to the a rays of radium C only ; while 

 the more intensely darkened centre was ascribable to the 

 influence of all those rays which range no further than those 

 of RaA. It was also found that calculation of these radii, 

 according to Bragg's law connecting the ionization range 

 with the square root of the molecular weight, gave distances 

 closely corresponding with those observed. The mica in 

 which these halos occurred was subsequently found to be a 

 lithia-bearing variety — the somewhat variable species classed 

 as zinnwaldite. But, as will be seen further on, this emen- 

 dation introduces no contradiction to the views originally 

 expressed. In a letter to 'Nature' of February 10th the 

 fact of the existence, in some cases, of an accentuated outer 

 border to the corona, in accordance with the increased effect 



* Phil. Mag. July 1908, p. 184. 

 t Communicated bv the Authors, 

 t Jolv. Phil. Mas-. February 1910. 



