62 
PROF. J. JOEY ON THE GENESIS OF PLEOCHROIC HALOES. 
attempt applied to this last faint halo, using a Leitz No. 4, gave 24:70 as the ratio 
of width of band to radius. This nearly agrees with 6 : 18 as above. The external 
radius of this halo was measured as 0 0151, and the internal radius as (P0104. These 
dimensions are subject to a small correction for the nucleus, which would leave the 
final readings as closely 0'0147 and O'OIOO : and the mean radius becomes 0'0123. In 
the same flake of Carlow mica and near the last halo a sharp but faintly coloured ring was 
investigated in the same manner ; the results were almost identical. Referring to the 
Table above we get (r—r x )/r = 5/19 nearly. The tabulated mean radius is (P01235. 
These minute haloes abound in the Carlow mica. In fact in places the clear mica 
is dusted over, as it were, with the black nuclear specks around which these haloes 
are formed, the delicate rings interlacing and overlapping in actual confusion. 
Some are, as already stated, so faint as to be hardly detectable. Others are well 
defined discs, the earliest formed part of the halo appearing as a darker border to 
the disc. One or two such growths are seen in the photographs already referred to. 
A vain endeavour was made to photograph the finer haloes but although many 
exposures and various combinations of lenses were tried nothing of value was obtained. 
The drawing (Plate 1, fig. l) to a magnification of 800 is as realistic as I can make it. 
A small correction has to be applied to these haloes for the effect of the nucleus in 
enlarging the radius. I have taken this as amounting to one-half the radius of the 
nucleus. This correction has been applied to the figures tabulated for r x and r. The 
estimated radius of the nucleus is given in the fourth column. 
These haloes are only well formed around very minute nuclei. Traces of them 
appearing round larger nuclei are not of value ; the nuclear correction becomes too 
large and uncertain. 
These haloes obviously correspond to the remarkable maximum of the curve of 
ionisation given, ante, fig. 1. They, therefore, must involve pre-eminently the ranges 
of U\ and U 2 . But they do not possess the quite correct radius called for by the 
curve. Converting the range in mica into corresponding ranges in air the first ring is 
found to be placed in the position shown above the curve. The darkened area 
(marked l) is a section of the ring halo. 
A discussion of this point cannot be entered on till further facts are considered. 
When the darkening within the first ring is yet only in its initial stages the outer 
shell due to RaC may begin to appear. This is seen in the published photographs .* 
The extreme radius of the outer ring at this stage is very little less than it finally 
becomes. Attention has already been called to the bearing of the early appearance 
of the effects of RaC upon views as to the mode of origin of the halo. 
Looking at the drawing, fig. 2, Plate 1, the first ring is seen almost lost in the 
darkening of the central region. This region has now become a “pupil.” Around it 
in the form of a delicate ring appears the “ second ring.” The second ring is rarely 
seen. I have found it but twice in the Ballyellen mica, although some scores of haloes 
* ‘ Proc. R. Dublin Soe.,’ loc. tit., figs. 5 and 6. 
