346 



POLARISATION. 



easily be shown by demonstration of the respective positions of the 

 three effective ellipses of elasticity. If, for instance, A A', BB', 

 and C C (Fig. 196) are the ellipses of the selenite plate and of the 

 two superposed plates of crystal in their sequence from below 

 upwards, and if, further, P P and N N are the polarising planes 

 (at right angles to each other) of the two Nicols ; then it is evident 

 from the figure that in the position indicated the homologous 

 planes of vibration of the ellipses follow one upon the other like 

 the steps of a spiral staircase, in so far as the alphabetical order 

 of the letters A B C or A B' C at the same time represents the 

 sequence of the corresponding axes in the direction of a spiral 

 turning to the left. This is the orthogonal consecutive position. If, 

 on the other hand, we take the pair of plates rotated to 90 

 (Fig. 197), C C falls between B B f and A A', and the vertices 

 A B C or A' B (J of the ellipses no longer appear as steps, but 

 make a zig-zag one above another a position which is denoted 

 as the alternative position, in contradistinction to the other one. 

 The difference of the colours which these positions produce is 

 then in many cases of such a nature that their comparison with 

 the acceleration and retardation colours affords the requisite data 



A' 



FIG. 198. 



for recognizing a given orthogonal position as a consecutive or 

 alternative one. When this is the case, the solution to the ques- 

 tion immediately follows, how the effective ellipse of elasticity in 

 the upper and in the lower plate is inclined. If, for instance, 

 Fig. 198 is recognized as a consecutive position, and Fig. 199 as an 

 alternative one, we know that the ellipse B B' belongs to the lower 

 and CO' to the upper plate. 



