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On distinguishing Biaxial and Uniaxial Micas 



7 m 



It is well known that where a plate of Iceland-spar, cnt with 

 faces perpendicular to the axis, is placed in a polariscope (the eye- 

 piece being '^crossed") and then rotated in its own plane, the 

 symmetrical cross and rings do not suffer any change of form or 

 posit ion J if, however, a plate of a biaxial crystal — as nitre — be 

 rotated in a similar manner, the dark bars which form the cross, 



as m 



fig, 1, will, when the plate has been rotated 45^, 



a 



out," and take the form and position represented in fig. 2. 



open 

 These 



1. 



2. 



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3, 



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facts and figures, though familiar to the student of optics, I here 

 repeat for the sake of the explanations which follow ; and this is 

 particularly important since observers of eminence have evidently 

 mistaken the optical character qf many of the so-called vniaxialot 

 hexagonal micas. When the micas referred to are examined by 

 my instrument I find the same evidence of a biaxial character. 

 The thin plates, if the line joining the poles is parallel to the 

 plane of primitive polarization, give to the eye symmetrical crosses 

 and rings, as in fig. 3, thai is, so nearly syniinetrical thai it is 



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