Age of Pleochroic Haloes. 647 



The amount o£ darkening o£ the mica produced by these 

 exposures was sensibly the same for A and D and for C and 

 E. B was slightly darker than C or E. The depth of 

 staining wns, therefore, very consistently dependent on the 

 exposure. In some places the areas overlapped a little, and 

 this part of the mica faithfully produced an effect due to 

 the added densities of the overlapping spots. 



A subsequent measurement of the thickness of these flakes 

 of mica, by focussing with a high power on the upper and 

 lower faces of the flakes as they rested on the stage of the 

 microscope, showed that the flake containing A, B, and C 

 was where thinnest 022 mm. thick, and the second flake, 

 containing D and E, was where thinnest 0*014 mm. thick. 

 The close agreement in the effects of the staining; on the two 

 flakes shows that the whole of the alpha rays must have been 

 absorbed in the second flake as well as in the first. 



The intensity of the darkening of the mica in these 

 experiments is such as to best admit of comparison with the 

 less developed haloes : those darkened uniformly out to the 

 range of Ra (0*0156 mm.), or RaF (0*0177 mm.), or Ra 

 Emanation (0*0196 mm.). The darkening is too faint to be 

 measured against the more exposed haloes which extend to 

 RaA or RaC. These are, indeed, generally almost opaque 

 centrally. Observations involving comparison of the halo with 

 the experimentally produced staining are, then, necessarily 

 restricted to the less exposed haloes, whose radial dimensions 

 may be assumed to be 0*016, 0*018, and 0*020 mm. 



From the experiments it may be deduced that the numbers 

 of rays falling upon circular areas of these radial dimensions 

 which will give the depth of staining of spots A, B, C, 

 will be : — 



Kadiusofhalo (H)16 0-017 0-018 0-020 



Intensity of A 296 X 106 337 X 106 370 X 10<3 466 X 10 a 



„ B 128 „ 146 „ 160 „ 202 „ 



„ C 120 „ 136 „ 150 „ 189 „ 



The observations on the haloes, designed to afford an 

 estimate of their age, must be directed, as already intimated, 

 to estimating the rate at which alpha rays are being emitted 

 by the nucleus. We have to consider also how the staining 

 effects produced by such radially emitted rays may be 

 brought into comparison with the parallel rays used in the 

 experiment. The cleavage flakes of mica used in the 



