320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
members of the uranium and thorium families. We know that each 
member of a family will exist in equilibrium amount within the 
nucleus possessing the parent element. Each alpha ray leaving the 
nucleus will just attam its range and then cease to affect the mica. 
Within the halo sphere there must be, therefore, the accumulated 
effects of the influences of all the rays. Hach has its own sphere of 
influence, and the spheres are all concentric. 
The radii in biotite of the several spheres are given in the following 
table: 
URANIUM FAMILY. THORIUM FAMILY. 
Millimeter. Millimeter. 
TReS OBI NAOT UN CUS yer eas ie ener 0.0330, || chonum Con 3:2 fees 4 ee RD 
PRR CIINDTINNA ACs tee eee rt ae ee AR FO2243 horn eAG pee ee eye . 026 
VATCINATA WOM asa eee eae OlIGaehwernta nations een ee 023 
HRALGUUNY His Sor eeeees SEAR AARS | ROEBe Oly 7 EhorkumeG]!. 224 ORE 30. etre . 022 
Radium: 9.250). Se Leen 98 TOLS6NC ThoinmeX. .. ep iasat tends . 020 
OMIT eae eee Seen O14 Radiothormimngss) ssseet eee 019 
Winamp lene ee OLS 74 Phorm 2 2 een 28 eee 013 
Wiramiinecaee ase conse eee 0118 
In the photograph (pl 1, fig. 2) we see a uranium and a thorium 
halo in the same crystal of mica. The mica is contained in a rock 
section and is cut across the cleavage. The effects of thorium C, 
are clearly shown as a lighter border surrounding the accumulated 
inner darkening due to the other thorium rays. The uranium halo (to 
the right) similarly shows the effects of radium C, but less distinetly. 
Haloes which are uniformly dark all over, as described above, are, 
in point of fact, ‘‘overexposed,” to borrow a familiar photographic 
term. Haloes are found which show much very beautiful internal 
detail. ‘Too vigorous action obscures this detail just as detail is lost 
in an overexposed photograph. We may again have ‘“‘underexposed” 
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 underexposed haloes we find circular dark 
marks having the radius 0.012 or 0.013 millimeter. 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 be- 
yond the vigorous effects of the two uranium rays. Then we find 
haloes carried out to the radii 0.016, 0.018, and 0.019 millimeter. 
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. Plate 2 
illustrates some of these stages, figure 2 of this plate being greatly 
enlarged to show clearly the halo sphere of radium A. 
