of several American Micas. 



341 



purpose, and with its aid we may hope for important advance^ 

 in our knowledge of the physical relations of the micas.* 



The instrument which I have used for the measurements 

 given in this paper is a modification of the goniometer of 

 Charles and Malus. It has a horizontal circle of about eight 

 inches diameter reading to minutes, with a tangent screw and 

 double readings. To the centre of the instrument has been 

 adapted a simple contrivance for holding two tourmalines, 

 and at the same time for securing the mica plate in the pro- 

 per position. The tourmalines have both a horizontal and 

 rotary movement, and are so arranged that the mica plate 

 can be conveniently held between them in an unvarying 

 position while the arm of the goniometer makes its revolu- 

 tion. The instrument is adjusted for use by bringing the 

 specimen into such a position that the line connecting the 

 optic axes shall be horizontal ; and by turning the arm of the 

 instrument through the requisite number of degi^ees, the two 



Mica Examined. 



Optic Angle. 



Muscovite of Grafton, .... 

 Phlogopite, Pope's Mills, 



,, Edwards, .... 



Biotite (?) Topsham, Me., 

 (Probably Phlogopite.^ 



Biotite, Monroe, IS. Y.,... 



Muscovite Royalston, Ms., 



„ Paris, Me., 



„ Brunswick, Me 

 „ Jones PallSjMd., 

 „ Philadelphia,... 



69° 3(y 

 7° 30' 

 15° 

 13° 30' 



67° 30' 



72° 30' 

 72° 30' 

 67° 

 60°30'-61° 



Colour, 



light brown, 



white glassy, ., 



brownish yellow, 



yellow brown, , 



deep reddish brown, , 



dark green, almost black 



dark brown, 



nearly colourless clear,.. 



light brown, 



dark green, 



banded in hexag'l figures 



Needle 

 deflected. 



19°-20° ' 

 12°-11°30 

 15° 

 16° 



13''-20° 



11° 



21° 



21°.21°30' 



21°.21°30' 



18°-19° 



21°36'-22' 



Percent.of 



rays trans> 



mitted. 



67-60 

 36-34-5 

 45 

 45 



39-36 



33 



63 



63-64-6 



63-64-5 



64-57 



64-6-66 



When the crystal was placed 80 that the rays of heat passed parallel to the 

 Optic axis (thus the Grafton mica was placed at an angle of 34° 30', the arrange- 

 ment remaining otherwise as before), the needle was, on repeated trials, de- 

 flected 24°, equal to 72 per cent, of all the rays passing while in the other posi- 

 tion (or with the normal parallel to the rays), only 60° passed. 



From these few trials (which are regarded as only preliminary and approxi- 

 mative), it will be seen that an interesting relation apparently subsists of the 

 sort looked for, and this last experiment is particularly worthy of confirmation 

 by extending it to numerous varieties. 



* Mr Blake presented his instrument and a series of measurements made 

 with it to the Physical Section of the American Association at the New Haven 

 meeting. 



