52 PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
haloes, although plainly representing an early stage of the uranium halo. Here again 
it appeared as if some unknown factor existed which could occasion different modes 
of origin for a halo. However, the idea that the larger embryos might be traceable 
to radium emanation acting as the parent element was found to afford a quite 
adequate explanation of the phenomenon. Additional evidence for this view is 
found in the fact that these haloes are almost invariably found located on conduits 
or veins in the mica ; conduits which undoubtedly conveyed radioactive materials at 
some past time. And even when we cannot demonstrate the existence of such a 
conduit we find in the linear arrangement of these haloes evidence that they have 
been generated along a crack or vein. The nature of the nucleus of these interesting 
haloes is not determinable. It is probably zircon. Whatever the mineral substance 
is, we must ascribe to it the power of absorbing or occluding the emanation and so 
becoming a centre of radiation of the a-rays of emanation and of the derived series 
of elements. 
I cannot claim to have been able to suggest explanations of every difficulty. 
Quite the contrary ; with increased number of observations fresh questions present 
themselves. These appear, so far, to be confined to the behaviour of the less 
penetrating a-rays of the uranium series of elements. 
I have included in this paper an attempt at an explanation of the reason why the 
halo develops as it does. It is not hard to show that primd facie the structural 
features of haloes are not what one would expect on theoretical grounds. By 
introducing the assumption that haloes partake of the properties of the latent 
photographic image and are capable of “ reversal ” or “ solarisation ” under certain 
conditions, it seems to be possible to explain the observed structural features. Quite 
lately I have found that a phenomenon is sometimes apparent in haloes which 
appears to set the possibility of solarisation or reversal beyond doubt. I have added 
a drawing of a reversed halo. 
Method of Measurement. 
From time to time considerable modifications have been introduced into the 
methods of effecting the measurement of haloes. The earlier observations were 
generally made with too low a magnification. There were also other causes of 
uncertainty. Much of the variation among the measurements disappeared under 
improved conditions of observation. But with all improvements the readings 
require care and practice. A Leitz micrometer eyepiece and a Leitz No. 4 objective 
give about the best magnification and conditions for reliable measurements. 
But the mode of using the micrometer is important. The usual practice in 
such cases is to traverse the image with the moving line of the micrometer, 
reading the micrometer head when the line is in diametrically extreme positions 
on the image. This method is defective for two reasons. It is difficult to secure 
any degree of accurate setting of the line when this is leaving the image. To 
