Haloes which are uniformly dark all over as described above are, in point of fact, “over-exposed”; to borrow a familiar photographic term.  Haloes are found which show much beautiful internal detail.  Too vigorous action obscures this detail just as detail is lost in an over-exposed photograph.  We may again have “under-exposed” haloes in which the action of the several rays is incomplete or in which the action of certain of the rays has left little if any trace.  Beginning at the most under-exposed haloes we find circular dark marks having the radius 0.012 or 0.013 mm.  These haloes are due to uranium, although their inner darkening is doubtless aided by the passage of rays which were too few to extend the darkening beyond the vigorous effects of the two uranium rays.  Then we find haloes carried out to the radii 0.016, 0.018 and 0.019 mm.  The last sometimes show very beautiful outer rings having radial dimensions such as would be produced by radium A and radium C. Finally we may have haloes in which interior detail is lost so far out as the radius due to emanation or radium A, while outside this floats the ring due to radium C. Certain variations of these effects may occur, marking, apparently, different stages of exposure.  Plates XXIII and XXIV (upper figure) illustrate some of these stages;

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the latter photograph being greatly enlarged to show clearly the halo-sphere of radium A.

In most of the cases mentioned above the structure evidently shows the existence of concentric spherical shells of darkened biotite.  This is a very interesting fact.  For it proves that in the mineral the alpha ray gives rise to the same increased ionisation towards the end of its range, as Bragg determined in the case of gases.  And we must conclude that the halo in every case grows in this manner.  A spherical shell of darkened biotite is first produced and the inner colouration is only effected as the more feeble ionisation along the track of the ray in course of ages gives rise to sufficient alteration of the mineral.  This more feeble ionisation is, near the nucleus, enhanced in its effects by the fact that there all the rays combine to increase the ionisation and, moreover, the several tracks are there crowded by the convergency to the centre.  Hence the most elementary haloes seldom show definite rings due to uranium, etc., but appear as embryonic disc-like markings.  The photographs illustrate many of the phases of halo development.

Rutherford succeeded in making a halo artificially by compressing into a capillary glass tube a quantity of the emanation of radium.  As the emanation decayed the various derived products came into existence and all the several alpha rays penetrated the glass, darkening the walls of the capillary out to the limit of the range of radium C in glass.  Plate XXV shows a magnified section of the

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