648 Profs. J. Joly and E. Rutherford on the 



observations to be presently recorded average from 0*016 to 

 0*018 mm. in thickness, thicker flakes being too dark for 

 convenient observation. The accompanying diagram of a 



Ftor. 1. 



RaLC 



flake 0*017 mm. in thickness, containing a nucleus placed 

 centrally in the flake and again near its surface, shows that 

 in the case of haloes extending radially 0*016 mm., the total 

 ionization is by no means included within the flake. Only 

 four out of the eight rays complete their ranges within the 

 semi-halo. The maxima of RaC and RaA are wholly 

 excluded. In the case of the centrally placed nucleus the 

 maxima of RaEm and RaF exert but little effect. These 

 excluded rays, however, affect the central area, where their 

 velocities are greatest and ionization effects least. It is also 

 seen that in cleavage flakes of this thickness as much as one- 

 half the entire effects of the rays may be removed with the 

 adjacent cleavage flakes. When the nucleus is at the surface 

 the hemisphere only is contained in the flake. Any accurate 

 allowance for these deficiencies is impossible, as they vary 

 with the position of the nucleus in the flake. There is a 

 small set off against them in the fact that the rays which in 

 the experiment darken the mica are not wholly included 



