70 
PROF. J. .TOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
doubtless, wider limits could be found. The pupil is simply the widening outwards 
of the halo under the influence of the ionisation, decreasing to the point of minimum 
effect. Outside the pupil the ring due to ThC 2 appears as a penumbra-like band. 
The accentuation of its extreme marginal boundary is, I think, less definite than in 
the case of the uranium halo. This may be due to the existence of a small number 
of rays which penetrate beyond the range of IlaCh. * 
Table YII.—-Dimensions of the Thorium Halo. 
No. 
n. 
r-2- 
’•3 (l). 
Pupil. 
R, 
Axis. 
Outside. 
Axis. 
Axis. 
Outside. 
Outside. 
1 
0-0106 
0-0115 
0-0168 
2 
— 
0-0115 
0-0174 
— 
— 
0-0402 
3 
o-oioo 
0-0113 
4 
■ 
0-0113 
0-0170 
5 
o-oioo 
0-0115 
0-0168 
6 
— 
0-0119 
0-0168 
7 
— 
0-0115 
0-0168 
8 
— 
0-0117 
0-0175 
9 
— 
0-0107 
0-0174 
10 
— 
0-0115 
0-0163 
11 
— 
0-0111 
0-0166 
0-0219 
— 
0-0407 
12 
— 
0-0113 
0-0165 
13 
— 
0-0117 
0-0176 
14 
— 
0-0111 
0-0173 
15 
— 
0-0112 
0-0170 
16 
— 
0-0110 
0-0170 
17 
— 
— 
— 
— 
0-0248 
0-0400 
18 
_ 
— 
— 
— 
0-0267 
0-0405 
19 
— 
— 
— 
— 
0-0256 
0-0411 
20 
— 
— 
— 
— 
0-0274 
0-0415 
' 21 
— 
— 
— 
— 
0-0281 
0-0422 
22 
— 
— 
— 
— 
0-0243 
0-0385 
23 
— 
— 
— 
— 
0-0250 
0-0393 
24 
— 
— 
— 
— 
0-0263 
0-0414 
25 
— 
— 
— 
— 
0-0248 
0-0392 
26 
— 
— 
— 
— 
0-0269 
0-0436 
27 
— 
— 
— 
— 
0-0231 
0-0402 
28 
— 
— 
— 
— 
0-0250 
0-0419 
29 
— 
— 
— 
— 
0-0273 
0-0415 
30 
— 
— 
— 
0-0255 
0-0408 
31 
— 
— 
0•0255 
0-0407 
Means 
0-0103 
0-0114 
0-0169 
0-0257 
0-0408 
Nuclear radius varies from 0 - 0006 to 0-0013 mm. 
Comparing the observed features of the thorium halo with those which might be 
expected if we assume the ionisation curve of fig. 5 to depict the development of the 
halo, we see that the first two rings are evidently accounted for by the maxima at 
* Rutherford and Wood, ‘Phil. Mag.,’ April, 1916. 
