68 
PEOF. J. JOLY ON THE GENESIS OF PLEOCHEOIC HALOES. 
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Fig. 5. 
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cm. 
Transformations.’ The ordinates for the ray ThC are reduced to one-third their 
full value, and those of ThC 3 to two-thirds their full value. This is done because it 
seems probable that ThC is transformed in 
two distinct ways, only one-third the 
transforming atoms giving rise to a-rays 
having the range 4‘8 cm. The remaining 
two-thirds, after losing a /3-ray, give rise 
to ThCo, which in breaking up emits rays 
of the range 8 '60 cm. 
The ionisation curve shown in fig. 5 is 
such a curve as a stream of parallel rays 
would give rise to. When we divide the 
ordinates by the squares of the abscissae, 
we net the curve fin. 6. This last curve 
O c> 
represents a very thin section of a halo 
taken through the centre of the halo- 
sphere, assuming that there was no in¬ 
fluence counteracting the effect of the outward spread of the rays. Finalty, fig. 7 
shows the last curve corrected for perspective in the case of a section taken 
symmetrically about the centre of the halo-sphere, and 4 cm. thick. 
We shall now see, just as in the case of the uranium halo, that, while the last two 
curves might well represent the final appearance of the thorium halo, they fail to 
represent the stages of its development. 
The first appearance of the thorium halo is that of two delicate rings, concentric 
one with the other. The area within the inner ring is generally more or less 
darkened. Occasionally, the darkening within the inner ring has obliterated the 
ring-like character of this first ring. That is, the first ring is merged in the general 
darkening. In other cases the ring shows a distinct band-like form, the radial 
width of the band being about (P002 mm. I have never seen this first ring 
unaccompanied by the second ring. The second ring is rather indefinite upon its 
inner and outer boundaries. It often possesses a central, or almost central, 
darkening. It is convenient to take the measurements to this central darkening, 
for it is the best defined feature of the second ring. A clear space intervenes between 
the first and second rings. Fig. 7, Plate 1, is a drawing of an embryonic thorium 
halo to a scale of 800 diameters. The outer radial dimension of the inner ring is 
closely 0'0114 mm., and the radial distance of the central darkening of the second ring 
is about 00169 mm. 
On a very few occasions a third ring has been seen. But its occurrence is so rare 
and its appearance so faint that I have little definite knowledge of it. One measure¬ 
ment placed the radial distance of its axis at 00219 mm. It may be of interest, 
however, to mention in connection with this ring that until I had sighted it the 
