PROF. J. -TOLY ON THE GENESIS OF PE KOCH ROIC HALOES. 
63 
sectioned in every plane must have been examined, i have found it twice in the 
Cornish granite. It would appear therefore to be more common in this granite, as of 
the number of haloes examined this is a much larger proportion. Finally, I recently 
found it in the Vosges granite. 
In the case of the Ballyellen mica it was got by making serial sections of a halo- 
rich crystal of haughtonite. The object in view at the time was to test the 
sphericity of the halo by examination of successive sections. Two of the sections 
passing near and through the centres of two overlapping haloes showed the hitherto 
undiscovered ring, beautifully developed. In the drawing I have endeavoured to 
sketch one of these haloes to a scale of 800 diameters. 
It will be seen that the new ring stands out specially dark and distinct. Its 
radial dimension has been the subject of many measurements. It has been read 
from 0'0172 to 0‘0177 mm. But the result given for the two Ballyellen haloes, 
0'0172, is for this mica very reliable. It appears to be referable to the prominence 
on the ionisation curve at the distance in air of 3'5 cm. from the centre. 
In other features there is nothing abnormal about these complex haloes. The 
appearance of r 3 is, however, an early one. They present a beautiful appearance. 
I give now the dimensions of the two Ballyellen haloes. The readings obtained 
were so much alike that it was found impossible to distinguish between them. The 
Vosges halo is a very thin section. The nucleus has left so indefinite a trace that it 
is difficult to be sure of the correction for it. I have taken a probable estimate. 
Table II.—Uranium Haloes showing the Second Ring. 
o o 
n. 
To. 
t’3- 
R. 
Internal. 
External. 
Axis. 
External. 
Axis. 
External. 
Internal. 
External. 
(1) 
(2) 
0-0113 
0-0150 
0-0147 
0-0172 
0-0172 
0-0188 
0-0185 
0-0203 
0•0205 
0-0217 
0-0221 
0-0287 
0-0327 
0-0327 
Nucleus of (1) and (2) O’0009 mm. radius; q.p. 
No. (1) is from Ballyellen, and (2) is from the Vosges granite. In the case of r x 
and R the external and internal radii are given. In the case of r 2 and r 3 the 
external and axial radii of the ring are recorded. The nuclear correction has been 
applied. 
It is not easy to account for the comparative rarity of these haloes. The second 
ring is, in all cases examined, accentuated in depth of colour. It is therefore not 
likely to escape notice. 
The outward expansion and darkening of the central region of the halo follows and 
is found in every stage. This is in keeping with the development of the halo under 
the rapidly declining ionisation ordinates characterising the shape of the curve 
