Crystals in the JSeighhourliood of an Optic Axis. 



95 



absorption is vanishingly small. In accordance with what 

 has been said above, these arrows also give the directions of 

 the axes majores for the ordinary, and of the axes minor es for 



the extraordinary waves on the neighbouring branches of the 

 hyperbolas. It will be seen that these directions are not 

 constant along the entire hyperbolic branches, but that they 

 become rotated through 45° during the passage through the 

 points Ci and 0/ of circular polarization. 



It is evident that in all these propositions the singular 

 axes play an extremely important part — they uniquely deter- 

 mine, from the qualitative point of view, the effects con- 

 sidered. 



8. Now as regards the appearances which may be noticed 

 in the neighbourhood of an optic axis, these present them- 

 selves in looking through a plate which has been cut in a 

 direction approximately normal to an optic axis. It is easy 

 to deduce the formulae for the intensities with a degree of 

 approximation corresponding to that of the experimental 

 observations — for a given incident light — made either with 

 the naked eye or by means of an inserted analyser. The 

 formula? are without exception very complicated, but may be 

 simplified for distances from the singular axes (\ and C/ 

 such that the terms containing the squares of the ratios € of 



