56 
PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
Now, according to figs. 2 and 3, the halo should appear with a much darkened 
central disc or pupil, this disc being surrounded by a relatively very faint hand or 
penumbra-like shading. For we see that the spreading of the rays, according to the 
inverse square law, has much reduced the maximum due to RaC. I am supposing 
now that we can translate the effects in air into effects in the mineral by simply 
shortening the ranges by the amounts proper to the greater stopping power of the 
mineral. That is, I assume that the curve defining the ionisation in air is followed 
in its general features in the mineral. 
Many haloes may indeed appear to conform fairly well with the distribution of 
ionisation shown in figs. 2 and 3. Photographs issued elsewhere show such haloes .* 
But, in point of fact, when we come to examine the halo in its earlier stages of 
development we find the appearance indicated by figs. 2 and 3 [ante) entirely 
departed from. Published photographs show that in its earlier stages the halo 
consists of concentric rings—or, more strictly, of shells—of varying density of colour, f 
In many respects we find that the curve fig. 1 very faithfully represents the 
development of the uranium halo. In the first stages we have a delicate ring which 
is clearly representative of, although not accurately agreeing with, the remarkable 
maximum which is placed at the distance in air of 2 '2 cm. from the centre or origin. 
Later we find this ring darkened up inside, and the first appearances of RaC 
presented as a faint outlying ring. But here there is plainly a discordance. The 
appearance of the shell due to RaC should be delayed till a much later stage in the 
development of the halo. Plainly, according to the ionisation curve, the central 
pupil should be carried out in air to a distance of about 4'5 cm. before the first 
indications of the outermost band appear. In the development of the region lying 
between the minimum of ionisation (at 4*8 cm.) and the embryonic ring two distinct 
shells may be detected. 
These facts of observation show that the uranium halo develops very much as if 
the inverse square law did nof operate at all or acted but partially, and it is evident 
that the final state of a halo according to fig. 1 is quite in accordance with the 
photographs referred to above. In a word, the development of the halo, as observed, 
indicates that for some reason the divergence of the rays which should lead to the 
obliteration of the features of the ionisation curve of fig. 1, is counteracted, and the 
maxima of the several rays, which contribute to the formation of the curve, sustained. 
I may add here that in the case of the development of the thorium halo the adherence 
of its genesis to the curve of ionisation derived from the integration of parallel rays 
is even more striking. 
I suggest the following explanation, although in itself invoking the aid of very 
obscure phenomena. It is known that the first effects of stimuli applied to the 
photographic plate are reversible by other stimuli. Wood has shown that these 
* ‘ Phil. Mag.,’ loc. at., Plate 8 ; 1 Proc. E. Dublin Soc.,’ XIII, Plate 3, figs. 2 and 3. 
t ‘ Proc. R. Dublin Soc.,’ loc. at., figs. 5 and 6. 
