Pleocltrolc Ualos. 635 



nssume that their average point of departure is about 2/3 

 the radius measured from the centre, we find that the distance, 

 0"0007 mm., traversed in the denser mineral (we will suppose 

 uraninite) is equivalent to 00011 in biotite, say. So far 

 there should be an additive correction of 0*0004 mm. But 

 the point of departure not being at the centre, but 0*0014 mm. 

 away from it, the subtractive correction exceeds the additive, 

 leaving finally a subtractive allowance of 0*0010 mm. The 

 correction is, probably, of approximately this amount. The 

 effect on measurements of outer radii is small. On inner and 

 much lesser radii it may be more serious. 



In certain minerals the nucleus itself takes a roughly 

 circular form and is opaque, and when the whole is blackened 

 up quite erroneous conclusions may be arrived at. Exami- 

 nation by reflected light will, in such cases, generally reveal 

 the central nucleus. Indeed, it is well to examine all much 

 darkened halos bv reflected light before forming conclusions 

 as to their radius. 



So far as our experience goes halos in cordierite or in 

 andalusite cannot be used for accurate measurements. Ap- 

 proximate measurements quite in accord with calculated 

 values can be obtained, but the boundary of the pale yellow 

 halos is too indefinite to permit of accuracy. Structural 

 features we have not traced in such halos. Cordierite is 

 much more sensitive to the a. radiation than biotite *. It is 

 possible that even a lesser quantity of a radioactive substance 

 could be detected in this medium than in biotite. The 

 greater sensitiveness probably leads to more rapid over 

 exposure and loss of detail. 



As an instance of the necessitv of making allowance for a 

 relatively large nucleus we may refer to the photograph of 

 a halo in biotite illustrating an earlier paper on this subjectf. 

 The measured diameter of this halo is 0*096 mm. The radius, 

 O048, however, will not agree with any calculated radius in 

 biotite, but it is found that the central mineral, in this 

 case zircon, is too large to be neglected. A number of 

 observations in different azimuths from the boundary of 

 the zircon to the periphery gives the radius as 0*039. This 

 is quite in harmony with the view that this is a thorium 

 halo. 



In the next table we have collected some measurements of 

 halos in various minerals. 



* Hiigge, Centrbl. f. Mm. 1909, p. (Jo. 

 t J oly . Phil. Mag. March 1907. 



