60 
PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
But in point of fact such borders have not been definitely identified. The sharp 
definition of the outer margin sometimes shown by a well darkened halo may be 
referable to such an effect of the nucleus. But the radius of the nucleus, in such 
haloes as are of importance to us, amounts to only 3 to 5 per cent, of the halo-radius. 
It seems more practical, therefore, to consider generally how much of this 3 or 5 per 
cent, might be deducted in order to obtain the true halo-radius or range which we 
seek to measure. Now we may take it that in developing haloes those radioactive 
particles which are placed on the outer surface of the nucleus will not sensibly affect 
the mica and we must make some assumption as to the depth in the nucleus from 
which those rays proceed which give us a visible effect, that is, which define that 
part of the boundary of the halo to which we bring the lines of the micrometer. 1 
take this depth as one-third the radius inward from the surface of the nucleus. If 
not quite accurate this assumption must at least reduce such error as must arise if we 
neglect the effects of the nucleus altogether. 
On this assumption, and further assuming that the stopping power of the nucleus 
is that of zircon, we have the effective defining ray advanced from the centre of the 
nucleus by f- v (the nuclear radius) and again brought back, as it were, by the greater 
retardation experienced in traversing the distance rj 3 in zircon. The loss of range 
due to the latter effect is r/6. Hence the net displacement outwards is r/2. 
Generally throughout this paper I have adopted this correction. I have not in all 
cases tabulated the nuclear measurements, for they are very uniform. 
The correction in the case of the emanation halo may require different treatment. 
In this case there is some reason to believe that the radioactive substances giving rise 
to the halo may have been occluded on the surface of the nucleus or had penetrated 
but a short distance inwards. In such a case the correction for the nuclear radius 
must be the full amount of this radius. This is the correction I have applied in the 
case of emanation haloes. I have tabulated the nuclear radius of these haloes, 
however, as they are somewhat abnormally large. Besides it is possible to urge 
reasons for a different treatment of the nuclear correction. 
The Uranium ITalo. 
The first beginning of the uranium halo is a delicate shell surrounding the nucleus 
and possessing an external radius which has been measured from 0'013 to O’OIS mm. 
The measurements given below have been made with much care. The nuclei in the 
case of these embryonic haloes are generally very small and in many cases exceedingly 
small. They may appear as minute black points in the centre of the band-like ring 
which constitutes the halo as seen in section. The black speck is sometimes larger 
than the nucleus, which with a high power may be made out as a refracting particle 
within it. The internal radius of the ring is lettered r x and the external radius r. 
I call this halo the first ling. 
It would be easy to cite many other readings in good agreement with these. 
