64 
PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
outside the point of maximum ionisation. When this outward growth has consider¬ 
ably developed and the region of the second ring is darkened up a sharp and narrow 
ring appears encircling the enlarged pupil. The photographs reproduced in the 
‘ Phil. Mag.,’ and ‘ Proc. It. Dublin Soc.,’ loc. cit., show this ring. The drawing, Plate 1, 
fig. 3, is from accurate measurements. Looking at the ionisation curve, fig. 1, we see 
only one feature which we can connect with it, the shoulder or prominence which 
is largely due to ltaA where the ionisation of this ray is a maximum. I call this 
the third ring. The following Table gives some careful readings of the dimensions 
of the third ring and of the pupil which lies within it. The outside radius of the 
third ring is r 3 ; its internal radius is r. The outside radius of the fourth ring is It : — 
Table III.—Haloes showing the Third King. 
Pupil. 
r. 
r 3 - 
R. 
1 
Carlow haughtonite. 
0■0205 
— 
0-0227 
0-0327 
2 
5' V . 
0-0207 
— 
0-0226 
0-0327 
3 
n " . 
0-0202 
— 
0-0228 
0-0330 
4 
n u . 
0-0185 
0-0196 
0-0221 
0-0329 
5 
0-0181 
— 
0-0189 
0-0325 
6 
0-0185 
— 
0-0193 
0-0325 
7 
Y osges .. 
0-0185 
— 
0-0228 
0-0331 
0-0193 
0-0216 
0-0328 
These results represent the normal type of halo showing the third ring. The 
average correction for nucleus is from (P0004 to 0'0006. The degree of general 
darkening in the above cited haloes is very various. No. 4 is much the darkest. 
Outside the third ring an annulus which very often is of quite unaltered mica 
appears. This evidently corresponds to the minimum of ionisation which begins at a 
distance of about 4*8 cm. from the origin of the curve, fig. 1. This annulus may be 
delicately shaded as it extends outwards, the darkening increasing almost imper¬ 
ceptibly, or it may be reduced to a mere chink by inward extension of the effect of 
RaC. The latter type is shown in the photograph reproduced in the ‘ Proc. K. 
Dublin Soc.,’ vol. 13, Plate 3, fig. 1. 
The appearance of the ring due to RaC, or the fourth ring, is at first a delicately 
shaded band, the shading being darker on the outer circumference. This corresponds 
to the steep descent of the Geiger or Bragg curve. Our opportunities of studying 
the distribution of ionisation along the track of a single a-ray in mica are restricted 
to the rings due to RaC and ThC 3 . There seems to be no doubt that the essential 
features of the ionisation curve in a gas are reproduced in the mica. 
Some outside radial dimensions of the fourth ring have been given above ; they 
are typical. The values 0'0327 or 00328 are the best supported of the values read 
in this mica of Co. Carlow. It is very much alike with the readings obtained in the 
Cornish mica, and in the mica of Vagnay in the Vosges. 
