16 



Mr. W. N. Bond on the Properties of 



objective was used throughout, in order to examine small 

 regions only of the crystal. 



It will be seen that, to a first approximation at any rate, 

 the plane containing the axes keeps tangential to the curve. 

 That is to say, that its direction relative to the crystal is 

 little changed by bending. 



Fig. 16 b is a composite diagram showing the form of 

 the dark bands exhibited by the specimen (PI. T. fig. 13} 

 in different successive orientations, the portion shaded being 

 irregular. (For some of the positions of the extinction-lines, 

 the corresponding direction of the planes of the nicols is 

 indicated by a small cross.) 



An experiment was next undertaken to see in what way 

 the plane changes direction as we pass across the crystal. 

 The microscope stage was slightly rotated till the two 

 branches of the dark brush observable in convergent polarized 

 light united to form a cross. The reading on the stage rotation 

 scale was then taken. This was repeated for different points 

 lying on a transverse line A A, fig. 16 b (crossing the bent 

 crystal twice). The results indicate a value of the change 

 in direction of the plane that is in moderate agreement with 

 the predicted results. The differences are probably due to 

 •the dark line not having a quite constant value of a at 

 different adjacent points along the length of the crystal, or 

 to the difficulty in making observations exactly along the 

 desired line AA. These would both affect the predicted 

 results. The measured results are plotted in fig. 17. The 



Fifif. 17. 



MX 





CRYSTAL 







eRVSTAU 



< 



r ""~ 









\\ 





I 







N 







I 







1 I 



i 







1 i 



i i 



! 1 



points lie along straight lines, which is to be expected, since 

 the dark extinction-lines are straight. Also the left-h;md 

 side yhlds an almost horizontal line, corresponding to the 

 dark line sloping very little on this side (see PI. 1. fig. 13). 

 But the slope of the extinction-line on the right-hand side, 



