PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
75 
factor is 2075. There is not perfect agreement. The conversion factor required to 
bring about agreement is sensibly larger than is required for the thorium haloes of 
the Vagnay mica. But this may be due to the assumption that the radioactive 
substance is carried on the surface of the nucleus. If the correction for the nucleus 
is taken as half the nuclear radius a fairly clo$e agreement is obtained between the 
range in air and in mica, assuming the conversion factor 2075. The difficulty 
remains in effecting this reconciliation that it is hard to imagine the sufficiently 
rapid absorption throughout the nucleus of the short-lived emanation. 
We shall next examine the uranium haloes of the Ballyellen haughtonite. The 
outer radius of well advanced haloes cannot differ much from 0 - 0334 mm. (Table IV). 
Dividing this into the range in air, i.e. into 6'94, we get the conversion factor as 
2077. This is in good agreement with the conversion factor of the Vagnay mica. 
Multiplying 0*0334 into 2075 we get for the limit of the uranium halo the feature 
numbered 5 in the ionisation curve, fig. 1. 
The next marked characteristic of the uranium curve is the minimum of ionisation 
at the distance 5*8 cm. in air. In Table IV we find that in well advanced haloes the 
pupil scales 0'0230. In these haloes we can seldom measure the nucleus. The value 
given, however, cannot be far wrong, for the outer radius of the third ring may 
extend to 0*0228 (Table III). Here the nuclear allowance is assured. We know 
that the final stage means a still further advance outwards. If now we take 0'0230 
and divide it into 4*8 cm., we get 2086 as the conversion factor. How closely this 
is in agreement with the ionisation curve in air appears when we apply the 
conversion factor 2075. This places the extreme development of the pupil in the 
position marked 4 in fig. 1. 
The external radius of the third ring is at 0*0216, about, according to the results 
of Table III. Applying the conversion factor 2075, we find the third ring located in 
the position numbered 3, fig. 1. Plainly it is in correspondence with the prominence 
on the curve at 4'3 or 4*4 cm. Using the same factor I give the location of the third 
ring as measured on the complex haloes of Table II. In this case we deal with the 
axial radius of the ring. It falls at 4*25 cm., and is evidently in agreement with the 
already determined position of the third ring. 
The second ring, which has only been found in a few haloes, but which is a 
perfectly clear and definite feature, is apparently associated with the prominence on 
the ionisation curve at 3*5 cm. If we apply to the axial radius of this ring (0*0172) 
the conversion factor 2075, we find it in the locus marked 2 in fig. 1. This is too 
much to the right. 
The last feature left to consider is the originating or first ring. It seems 
impossible to disassociate this ring from the marked ionisation maximum of the 
integral ionisation curve. As the result of very many measurements it has been 
found that the outer edge of the ring is 0*0142 mm. from the centre and the inner 
0*0105 mm. The latter radius can hardly claim to be as accurately determined as 
vol. ccxvir.—A. 
M 
