PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
67 
according to which the halo in air and the halo in mica are compared are explained 
later. Fig. 6 is a drawing of the ring halo due to emanation to a scale of 
800 diameters. 
As already referred to, there is interesting evidence as to the origin of these 
haloes. They are—almost without exception—found located on cracks or veins in 
the mica. I have in another place* referred to the conduits in this mica. These 
conduits undoubtedly contained radioactive materials. They are bordered by 
radioactive staining in a manner resembling the artificial halo of Rutherford, 
which also is an emanation halo. The border may be very faint or very dark. Its 
radial extension is in general very much the same as that of the first ring of the 
uranium halo. And in many cases we can quite easily detect that the border is 
due to a succession of minute haloes whose centres are set close together along 
the cgnduit. The border in these cases consists, in fact, of a number of overlapping- 
circles or rings, having approximately the radius of the first ring. 
But every here and there one of the larger rings—the originating ring of the 
emanation halo—is found among the others. Examination shows in such cases a 
nucleus extending into the conduit, consisting of a refracting particle. Some of 
these emanation rings may be attended with the ring of RaC and other features of 
the more advanced halo. Again, the emanation halo may occur upon a fine, 
hair-like crack in the mica, and otherwise unattended. Or we may find them 
ranged in a linear sequence, with the originating conduit barely detectable. 
Nor is this the only evidence for the movement of radioactive materials in this 
mica. It is common to observe near the margin of a flake of the mineral, darkened 
veins, sometimes plainly following structural features of the mica, and branching at 
definite angles. Shapeless blotches of staining may attend these arborescent 
growths. Or the field may be strewn with elongated, sausage-shaped objects 
deeply stained. These appear to result from the coalescence of small spherical 
haloes, having the radius of the first ring, from 0‘0137 to 0'0141 mm. These objects 
may attain the radial dimensions of more developed haloes. Thus, there may be an 
inner dark cylindrical core of radius 0'0190, and an outer cylindrical penumbra 
scaling 0-0327 mm. The entire structure may in some cases result from the entry of 
radioactive materials into a crack of short length, or from a succession of linearly 
arranged and closely approximating radioactive nuclei. 
The Thorium Halo. 
If we investigate the formation of the thorium halo, assuming that a Geiger curve 
defines the ionisation due to each ray, in the manner already described in the case of 
the uranium halo, we obtain, as the resultant curve of ionisation due to the seven 
rays which go to form the halo, the curve given in fig. 5. The scale is the same as 
that of fig. 1. The ranges are those cited in Rutherford s ‘ Radioactive 
* Huxley Lecture at the University of Birmingham, 1912. 
VOL. COXVI1.—A. L 
