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PROFESSOR POWELL ON THE ELLIPTIC POLARIZATION OF LIGHT. 



Postscript. 



Received May 8, 1845. 



Since the foregoing paper was communicated I have obtained some additional in- 

 stances of elliptic polarization, which may be not uninteresting in connexion with 

 those therein mentioned. 



Besides plumbago (in which the proportion of metal present is at least very vari- 

 able and small) I have now observed a slight, but quite unequivocal ellipticity in the 

 reflexion from China ink, which is wholly non-metallic. It is at its maximum at an 

 incidence of 62°, and its azimuth of restoration by the rhomb is about 5°. 



I have examined also chromate of lead, which gives very small ellipticity, which is 

 at a maximum at an incidence of 70° (agreeing well with the mean refractive index), 

 and the azimuth of the rhomb is about 6°. 



