74 
PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
Table VIII. 
Pupil radius. 
R. 
0-0215 
0-0323 
0-0212 
0-0327 
0-0217 
0-0332 
0-0230 
0-0336 
0-0230 
0-0319 
0-0204 
0-0332 
0-0219 
0-0326 
0-0190 
0-0327 
0-0215 
0-0328 
These results do not differ from the dimensions read in the Co. Carlow mica. We 
must compare them with the haloes of Table III (ante) rather than with those of 
Table IV, in which the ionisation has gone so far that the nucleus is no longer 
visible. The mean results are practically identical with those of Ballyellen. For 
some reason the Vosges uranium haloes do not exhibit the detail of the Carlow 
haloes. I have only once seen the third ring separated from the pupil in the former 
mica. The haloes of the Table given above and those of Table III appear to be in 
the same stage of development, I conclude that the conversion factor which applies 
to the Vosges mica applies also to the Ballyellen mineral. 
We shall next examine the emanation halo. The counterpart of this halo in air 
exhibits a conspicuous maximum, which may be taken to attain its highest point at a 
distance from the centre of 3'35 cm. 
The ring-shaped embryonic halo, which I have explained as due to nuclei occluding 
emanation of radium, measures on its outside radius in haughtonite 0"01734 mm., 
and on its inner radius (P0142 mm. The mean radius is (P0157. Upon these figures 
I venture to place considerable confidence. They have been carefully checked. The 
outside dimension of this halo is particularly easy to measure. It is generally sharp 
in outline. The inner radius is less easy to deal with, but as each reading in the 
Table is the mean of two or more readings which do not differ much among 
themselves, I think that in the general mean confidence may be placed. 
When the mean radius as above is divided into 3 - 35 cm. we get the conversion 
factor as 2134. As regards the outlying part of the curve, that due to RaC, it is 
sufficient to say that the readings applying to it differ in no way from those obtained 
from the ordinary uranium halo. The same remark may be made to intervening- 
features. But it is not always possible to say whether a particular halo has started 
with the emanation or with uranium. 
I have given already (fig. 4) the ionisation curve in air proper to this halo, with 
the generating halo-ring marked in its correct position, assuming that the conversion 
