Crystals i/i the Neighbourhood of art Optic Axis. 97 



of the incident light is rotated— changes which take place 

 precisely in the manner indicated by the formulas. When 

 natural incident light is used, then according to the strictly 

 correct formulas, the rings are not entirely absent, but, de- 

 pending as they do on e 2 , are faintly outlined only in the 

 immediate neighbourhood, of the singular axes. As a matter 

 of fact there is in this case visible nothing beyond a mere 

 trace, which appears as a dark spot, of the first minimum 

 within the Brewster's pencils. 



10. Although, in accordance with what has been said 

 above, the interference rings observable with onlv a single 

 polarizer (idiophanic rings) prove the propagation of two 

 similarly rotating elliptic waves, they do not exhibit the 

 change in the direction of rotation during the passage 

 through the f-axis, referred to in the fourth proposition. A 

 supplementary experiment thus appears desirable, and the 

 following arrangement would appear to meet all requirements. 



If instead of plane polarized we use elliptically polarized 

 incident light, and view it with the naked eye, then according 

 to theory, and as is otherwise evident, the phenomenon to be 

 observed must assume a certain dissymmetry, which was non- 

 existent with plane polarized incident light. Now this want 

 of symmetry is, as a matter of fact, very marked. 



If we use, say, a plate of rhombic andalusite (whereby the 

 plane Ajf coincides with the plane of optic axes A, A 2 ) in v\ hite 

 or, perhaps better, blue light, and if the plane of the polarizer 

 be normal to that of the optic axes of the crystal, then we 

 obtain a nearly circular system of rings, crossed by a dark 

 band which lies in the plane of the optic axes, a pheno- 

 menon with two mutually normal lines of symmetry. If 

 between the polarizer and. the crystal we insert a quarter- 

 wave plate, then as the latter is rotated from the position of 

 vanishing effect, the appearance becomes entirely asymmetric, 

 and in a manner which agrees with the predictions of theory. 



The dissymmetry is particularly striking when the light 

 incident on the plate is right or left-handed circularly polarized 

 light. In this case there appears on one side of the plane of 

 the optic axis (the f-axis in fig. 3) a strongly marked dark 

 spot, and on the other a bright spot, and these spots change 

 places if the direction of rotation is reversed. 



This observation proves in the simplest manner possible 

 that the waves propagated through the crystal on the two 

 sides of the £-axis have opposite directions of rotation, thus 

 furnishing the desired and necessary completion of the proof 

 of the theory. 



Gottingen, March 1902. 

 Phil. Mag. S. 6. Vol. I. No. 19. July 1902. H 



