PLEOCHROIC HALOES—JOLY. 323 
scale in the halo. Initial recombination seems, however, to be 
absent or diminished in effectiveness; probably because of the new .- 
stability instantly assumed by the ionized atoms. 
One of the most interesting points about the halo remains to be 
referred to. The halo is always uniformly darkened all round its 
circumference and is perfectly spherical. Sections, whether taken 
in the plane of cleavage of the mica or across it, show the same 
exactly circular form, and the same radius. Of course, if there was 
any appreciable increase of range along or across the cleavage the 
form of the halo on the section across the cleavage skould be elliptical. 
The fact that there is no measurable ellipticity is, I think, one which 
would not on first consideration be expected. 
For what are the conditions attending the passage of the ray in 
a medium such as mica? According to crystallographic conceptions 
we have here an orderly arrangement of molecules, the units com- 
posing the crystal being alike in mass, geometrically spaced, and 
polarized as regards the attractions they exert one upon another. 
Mica, more especially, has the cleavage phenomenon developed to 
a degree which transcends its development in any other known 
substance. We can cleave it and again cleave it till its flakes float 
in the air, and we may yet go on cleaving it by special means till 
the flakes no longer reflect visible light. And not less remarkable is 
the uniplanar nature of its cleavage. There is little cleavage in any 
plane but the one, although it is easy to show that the molecules 
in the plane of the flake are in orderly arrangement and are more 
easily parted in some directions than in others. In such a medium 
beyond all others we must look with surprise upon the perfect sphere 
struck out by the alpha rays, because it seems certain’ that the 
cleavage is due to lesser attraction, and, probably, further spacing 
of the molecules, in a direction perpendicular to the cleavage. 
It may turn out that the spacing of the molecules will influence 
but little the average number per unit distance encountered by rays 
moving in divergent paths. If this is so we seem left to conclude 
that in spite of its unequal and polarized attractions there is equal 
retardation and equal ionization in the molecule in whatever direc- 
tion it is approached. Or, again, if the encounters indeed differ in 
number, then some compensating effect must exist whereby a direc- 
tion of lesser linear density involves greater stopping power in the 
molecule encountered, and vice versa. 
The nature of the change produced by the alpha rays is unknown. 
But the formation of the halo is not, at least in its earlier stages, 
attended by destruction of the crystallographic and optical proper- 
ties of the medium. The optical properties are unaltered in nature 
but increased in intensity. This applies till the halo has become 
so darkened that light is no longer transmitted under the condi- 
