342 Dr B. Siliimaii, Junior's Optical Examinatiojt 



series of coloured rings of a common binaxial mica will come 

 successively into view. A vertical cord placed in an open 

 window is required to complete the arrangement ; the instru- 

 ment is so adjusted, that the cord accurately intersects the 

 black dots of the inner coloured circle about one axis ; a 

 revolution is then made till the cord intersects in the same 

 manner the other axis ; the amount or angle of this revo- 

 lution is the angle between the axes. With this arrangement 

 there is no difficulty, after a little practice, in obtaining a 

 series of measurements on the same specimen, varying from 

 each other but a few minutes at most, without having re- 

 course to lenses or other means of more accurately defining 

 the field of observation or reducing the area of the coloured 

 circles. Such modes of greater accuracy are important for 

 the more delicate physical questions previously suggested ; 

 but for the purpose of mineralogical determination, the means 

 just described are quite sufficient, since it is shewn that in a 

 series of specimens from the same locality there is generally 

 a difference of angle greater than any error of observation 

 arising from the imperfection of the instrument employed. 



Additional interest is given to this inquiry from a compa- 

 rison of the chemical relations of the various species of mica 

 and their corresponding differences in optical characters. 

 For this reason we briefly recapitulate the divisions which 

 are adopted by Professor Dana in the late edition of his 

 System, and which are also given, with a recapitulation of 

 the chemical formulas, on page 118 of vol. x. of Silliman's 

 Journal. The species of mica now recognised are muscovite, 

 margarodite, emerylite, eupliyllite, margarite, lepidolite, phlo- 

 gopite, and biotite. Of these, all but the last are binaxial. 

 Our observations will be confined mainly to muscovite, lepi- 

 dolite, phlogopite, and biotite. 



1. Muscovite. — This name has been proposed by Dana to 

 embrace those binaxial micas whose angle of polarization is 

 between 55° and 75°, excepting however the lithia micas, 

 which, having a peculiar composition and a very high angle, 

 are included under the species lepidolite. The terms " oblique 

 mica," "common mica,'' and "binaxial mica," formerly ap- 

 plied to this species, now fail to be distinctive, since we have 



