DETERMINATION OF THE AXES OF ELASTICITY. 



335- 



If we place two double-refracting plates of equal thickness and 

 with equal effective ellipses of elasticity together, in such a manner 

 that they intersect at right angles (Fig. 191), they act as a single- 

 refracting medium. The undulations 

 of light which take place in the 

 lower plate parallel to the major 

 axis coincide in the upper plate 

 with the plane of the minor axis 

 and conversely. The ratio of the 

 velocities with which the two systems 

 of waves traverse the lower plate is 

 consequently inverted on entering 

 the upper one, and the difference of 

 phase attained is therefore elimi- 

 nated during the passage through, 

 than the other, by the magnitude d, the combined effect would 

 clearly be the same as if a single plate of the thickness d were 

 observed. 



The same method of reasoning may be applied also to plates of 

 any thickness and of any ellipse of elasticity. The effects will 

 always be added together if the two ellipses are equally situated, 

 and become partially or entirely cancelled if they intersect at right 

 angles. Let us assume, for instance, that the interference colour of 

 the one plate is red of the first order, and that of the other yellow 

 of the first order (of course, on the supposition that the position is 



FIG. 191. 

 If the one plate were thicker 



