PROF. J. -TOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
65 
The final stage of growth of the uranium halo may be taken as that at which the 
outward expansion of the pupil has absorbed the third ring. The halo at this stage 
very often exhibits the appearance of a large and very dark pupil surrounded by an 
almost uniform penumbra-like band representing the effects of RaC. The last four 
haloes in the Table given below are of this character. In the other five haloes the 
darkening of the external ring has not extended inwards to meet the pupil. Its 
inner radius is denoted by R,. The haloes in this Table may be described as showing 
the final state of a uranium halo before every feature disappears in blackening which 
extends out to the limits attained by RaC. 
The measurements contained in the Table are not quite so available as those 
already given, on account of the fact that the correction for the nucleus must be 
guess-work. I have made an allowance for it amounting to 0‘0006 mm., that is I 
have assumed the nuclear radius to be (P0012 mm. 
Table IY.—Uranium Haloes : Final Stage. 
Pupil. 
Ri. 
R. 
1 
Carlow. 
0-0230 
0-0282 
0-0340 
2 
0-0230 
0-0279 
0-0341 
3 
5?. 
0-0226 
0-0285 
0-0340 
4 
. 
0-0224 
0-0285 
0-0334 
5 
.. 
0-0214 
0-0273 
0-0335 
6 
n. 
0-0221 
— 
0-0327 
7 
0-0225 
— 
0-0330 
8 
Vosges. 
0-0215 
— 
0-0323 
9 
? J ' ’ 1 * . . 
0-0221 
— 
0-0327 
0-0223 
0-0333 
This final stage is depicted in the drawing, Plate 1, fig. 4. It will also be found 
in the photographs already referred to. 
The Emanation Halo. 
Haloes are found in considerable numbers in the Carlow mica, which consist of a 
solitary ring showing no detail whatever within, and which are only clearly distin¬ 
guished from the first ring of the uranium halo by their dimensions. They vary in 
appearance much in the same manner as does the embryonic uranium halo. That is 
to say, some show a greater degree of darkening than others. They may present the 
appearance of a delicate smoke-coloured ring, or be more or less darkened within, 
the original ring appearing as a still darker border. They are often wonderfully 
sharp in outline. At an early stage they may show the faint development of the 
ring due to RaC, that is, of the fourth ring of the uranium halo. This fact at once 
attaches them to the uranium series of elements. 
