6 On distinguishing Biaxial and Uniaxial Micas. 



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Abt. II. — On a Method for distinguishing bettaeen Biaxial 

 and Uniaxial Crystals when in thin plates^ — and the results 

 of the examination of several supposed Uniaxial Micas ; by 

 W. P. Blake. (Read before the American Association for the 

 Advancement of SciencCj at Cincinnati, May. 1S5L) 



The greater number of the "black micas" so-called, have a deep 



olive green or "bottle green" color, so deep as generally to be 



opaqne when the thickness exceeds one-fourth of a millimetre.* 

 The other colors most frequently observed are a dull brownish 

 green, sometimes yellowish, and a fiery red. Of course their 

 power of absorbing light varies greatly j thus, the mica from 



Greenwood Furnace, olive ^een, is opaque in plat-es over '500 ram. thick 



Sussex Co., K J., (No. 115) bottle green, "■ " « " ' -222 *' 



St. Lawrence Co., (No. 116) browni.-?h " « " « « -062 " 



(No. 113) olive green, « " " « -042 " 



These thicknesses were taken with an instrument constructed 

 by Soleil, for measuring the thickness of thin plafes.f It is called 

 a spherometer and was designed by M. Cauchoix. 



In stating the color of specimens, I have given that color which 

 they present when a plate sufficiently thin to be transparent is 

 held near the eye, while the light from the clouds or window is 

 allowed to pass through it. It is not therefore possible to exam- 

 ine plates of ihese micas in the ordinary way, even when the 

 thickness is less than stated ahoi^e ; and of course when so very, 

 thin, the images formed around the pole or poles of the result- 

 ant axes, are much extended and it would generally be impossible 

 to distinguish them or the dark bars which cross the center ; and 

 when they can be seen, they so nearly resemble the symmetrical 

 image formed in uniaxial crystals that they cannot be distin- 

 guished from them. 



In order to enable me to give these dark micas a more com- 

 plete and thorough examination, I have arranged, in addition to 

 the combination of lenses and ^' Nichols Prism," which 1 described 

 at the meeting of the American Association last August, a large 

 convex lens, placed between the polarizing mirror of my instru- 

 ment and the film of mineral to be examined; I also use, instead 

 of the variable and uncertain light from the clouds, a broad gas 

 flame, which I bring near to the mirror, so that its reflected image 

 is condensed by the large convex lens upon the micaj with this 

 arrangement I have been able to discern the rings and cross in 

 plates of the dark micas much thicker than could be examined by 

 any former arrangement. It now became interesting to examine 

 some of the micas whose optical uniaxial character was equivo- 

 cal, and for this purpose I adopted the following test. 



Ordinary visiting cards are from '3 to A of 1 mm. thick, 

 f Described in E. Peclet's Traite de Physique, 4e ei Paris, 1847, i, 5, 



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