14 Mr. W. N. Bond on the Properties oj 



The angle of slope is, however, as nearly as can be 

 estimated, of" constant value. When the sense is changing, 

 an intermediate portion may show a double dark extinction- 

 line or cross. Or the sense may change half-way radially 

 across the crystal, causing a " V " shaped band. 



To a first approximation, the band is at 45° to the radius 

 of the are formed by the crystal. It is difficult to measure 

 this angle accurately, and it is possible that it is not quite 

 constant. 



On the other hand, it will be seen, on looking at the 

 figures in the following table, that for all radii of curvature 

 and widths of crystal examined, the slope is not greatly 

 removed from 45°. 





Table II 



• 





Eadius of Curva- 

 ture. 



Width of 

 Crystal. 



Radius 

 Width * 



Slope of Extinc- 

 tion-line in degrees. 



17 



21 



8-1 



44 



30 



2-0 



15 



64 



17 



1-8 



9-45 



60 



10 



1-6 



G-25 



53 



7*5 



2 



3*75 



55 to 64 



10 



2 



5 



50 



7-5 



1-6 



469 



40 



If these measurements be used to plot the slope of the 

 extinction-line against „.. , , , no connexion between them 



can be found. 



Some of the crystals were bent in such a direction that the 

 plane containing the optic axes was at right angles to the 

 plane of bending. 



The plane containing the optic axes is parallel to the 

 principal section of one of the nicols at all points where 

 -extinction is occurring on any one dark extinction-line AB 

 (fig. 15). Thus the angle between this plane at any point 

 and the radius to that point would vary with the distance of 

 the point from the inner edge of the curve. Let a. be the 

 slope of the dark line as indicated in fig. 15. Then the 

 change in direction of the plane containing the optic axes 

 on passing radially from B to is equal to the change in 

 direction on passing from A to 0, and is thus also equal to 

 the angle subtended by the extinction-line at the centre of 



