1821.] Crystallized Bodies on Homogeneous Light. 127 



Here the virtual pole was coincident with the fifth maximum 

 (or thereabouts) of the extraordinary ray from the pole P ; the 

 succession of tints, however, unless close to the virtual pole, is 

 omitted, in order to shorten the table. 



Table YI.—'Rochelle Salt, Thickness = 0-08557 in. 



In this plate the virtual pole fell about half way between the 

 fourth maximum and the fifth minimum of the extraordinary ray 

 from the apparent pole P for the mean red rays. When once 

 more ground down, it gave as follows : 



