76 
PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
the former. It is more difficult to measure. But it will be noticed that the ratio 
(r—r T )/?’, derived from the Table, is in agreement with the results of investigation with 
use of high magnification as referred to on p. 61. 
The integral ionisation curve, which has been very carefully plotted, would refer 
the axis of the ring to a radial distance of 2 "2 cm. from the centre of the halo. 
But the axis of the first ring in mica is at a radial distance of 0'0124 mm. from the 
halo-centre. Dividing this into 2‘2 cm. we get for the conversion factor 1774. 
The first ring is, then, too great in radius to fit the curve of integral ionisation in 
air. Nor do I believe it possible by any allowance for the nucleus or refinement on 
the measurements to bring them into agreement. 
Taking the conversion factor as 2075, which, as we have seen, is supported by 
Bragg’s Law and by every feature of the thorium halo, and approximately by 
several of the uranium halo, we find the first ring to be located in position 1 in fig. 1. 
The conspicuous and abundant presence of this ring forbids us to ascribe it to any 
minor feature of the ionisation curve. Nor have we any other feature to which to 
refer the origination of the halo. It is plainly a primary feature. And within the 
boundary of this ring no trace of any other distinct feature has been observed. 
Irregular staining has been seen occasionally, adjacent to the nucleus, but such is not 
alone variable and irregular in dimensions.; it may be, and generally is, entirely absent. 
Conclusion. 
As the result of a very large number of measurements we find that the structure 
of the halo is determined by the added ionisation effects of all the a-rays concerned 
in its genesis. A simple addition of the ordinates representing the ionisation of each 
ray serves to define the location of every feature of the thorium halo. Not one 
feature of this halo has been discovered which may not be referred to the features of 
the integral curve of ionisation proper to this family of radioactive elements. The 
relative spacing of the features of the halo fits with satisfactory accuracy the features 
of the curve. The only criticism here is that the first and second rings are a little 
too widely separated. But the departure from perfect fit is so smaU that we would 
not be justified in laying any stress upon it unless we were assured of a higher order 
of accuracy in all the constituent elements entering into the matter than we are at 
present entitled to assume. 
This close interfit of the halo in air and the halo in mica gives us a conversion 
factor which agrees very nearly with that derived independently on the additive law 
discovered by Bragg for a mica which exhibits very similar stopping power to that 
containing the thorium haloes. 
Applying this conversion factor to the ionisation curve for uranium haloes we 
find, indeed, that the outer features of the uranium halo exhibit fairly satisfactory 
interfit with the halo in air. But the inner features do not. The first 
and second rings appear to be displaced outwards. Both these features are 
