PROF. J. JOLY ON THE GENESIS OF PLEOCHROIC HALOES. 
57 
stimuli will only reverse one another’s effects if applied in a certain order. Thus the 
latent image due to friction or pressure is obliterated by any other known form of 
stimulus such as X-rays or light. Next in order of stability is the latent image due 
to X-rays. Light action will remove this. A very brief light shock or flash comes 
next. This can only be reversed by a long continued light stimulus. In this list the 
stimulus which produces the less stable system is unable to reverse the more stable 
effectsA It does not appear improbable that in the mineral the effects of the a-ray 
may correspond in character to photographic latent effects, that is, to effects of an 
incomplete character, no definite molecular rearrangement being accomplished as the 
result of ionisation and liberation of S-particles. There is not wanting independent 
evidence that this is what actually happens. It is known that the halo can be 
obliterated by heating the mica. This fact led the earlier observers into the curious 
notion that the halo was an “ organic ” effect. Presumably the idea was that it was 
a coloration due to carbon which became oxidised upon heat being applied. If the 
halo was due to the formation of a stable oxide of iron it seems certain that even a 
red heat would not affect it. And it is said that some haloes—very strongly 
developed ones—refuse to be dissipated by heat. Here we have what I think may 
be described as a reversing effect due to heat. Again the halo shows by its optical 
behaviour that it is in crystallographic continuity with the rest of the mica. There 
has apparently been some intermolecular disturbance or strain set up by the ionising 
actions generating the halo, so that the light absorption towards a polarised ray is 
increased. But it continues to exhibit the optical properties of the original medium, 
and, presumably, retains much of the original structure. I shall assume that just as 
it can be reversed by intense heat so the coloration of the halo may be obliterated or 
reversed by other stimuli when these are of a certain kind, or that it may be so 
reversed at some stages of its genesis. 
The effects going on in the part of the mica traversed by the rays are highly 
complex. The character of the ionisation due to each single a-ray, as shown by the 
Bragg curve, suggests that at points along its path the ionising stimulus may be 
regarded as varying with the velocity of the ray. And we see that every part of the 
field around the nucleus, except the outermost region which is traversed by the rays 
of RaC or ThC 3 only, is exposed to the passage of rays of very varying speeds. 
I assume that, within certain limits of distance and intensity, some of these 
stimuli are able to reverse the effects of prior stimuli. The successive effects may be 
separated by any time interval, but the space interval is limited, although dimensions 
considerably greater than molecular are involved. Moreover, there must be a certain 
order maintained in the succession of stimuli of different velocity just as in Wood’s 
experiments. 
The chance of a ray passing within a certain effective distance from any point at 
the radial distance r from the centre is, say, _p/r 2 . For two rays to pass within this 
* Wood, ‘Phil. Mag.’ [6], pp. 577-587 (190,3). 
