72 
PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
to the position of this halo in the many slides in my possession was lost during the 
military occupation of Trinity College (and of my Laboratory) during the Sinn Fein 
rising in Dublin. I had intended obtaining a photograph of this halo or making a 
measured drawing of it. 
It is probable that the scarcity of these compound haloes is to be referred to the 
generally great predominance of the thorium present, which has the effect of masking 
the effects of the uranium. This explanation is, however, not satisfactory in all 
respects. 
The Conversion Factor. 
By this term I refer to the number which multiplied into the range in mica 
affords the equivalent range in air at pressure of 760 mm. and temperature of 
15° C. The importance of this number is considerable. With it is involved the 
most interesting questions arising from the study of haloes. 
We possess two methods of finding the range in a mineral equivalent to the range 
in air. We may calculate it on the basis of the chemical composition and density of 
the mineral according to laws determined by Bragg and Kleeman. Or we may 
determine it for any particular range if we are justified in identifying some feature 
of the halo as the result of a ray whose range in air is known. 
Taking first the method by Bragg’s Law, we find # that 1 cm. in air of density 
0'0012 corresponds to a range in the haughtonite of Co. Carlow of (P000473 cm. 
This result is based on a chemical analysis of this mica, and a careful determination 
of its density. A small correction may be made to bring it into comparison with the 
tabulated ranges in air of the temperature 15° C. The density is (P00122 at this 
temperature and at standard pressure. The equivalent range then becomes 
(P000482 cm. The reciprocal of this number gives the factor which multiplied into 
the range in the mineral gives the range in air. We may write the conversion 
factor as 
2075. (1) 
When we refer to the halo itself for the conversion factor we assume that the 
connection between the ionisation and the velocity is the same in the mica as it is 
in a gas in so far that the maximum of ionisation is attained in both cases when the 
velocity has fallen to the same fraction of its original value. 
Beginning with the integral curve of ionisation for the thorium family of elements, 
and comparing it with the measurements of thorium haloes in various stages, we find 
on the curve, fig. 5, two very prominent maxima at small radial distances from the 
centre. Outside the second appears a steep decline of the ionisation curve with two 
steps near its upper part and a pronounced step lower down. Then we have a 
minimum of ionisation which rises outwards to the blunt maximum due to ThCh. 
In Table VII we find first a ring or band of a radial width about one-fifth the 
* ‘Phil. Mag.,’ April, 1910, p. 631. 
