Optical Examination of several American Micas. 373 
‘well adapted for this purpose, and with its aid we, may hope for ~ “ag 
nt x advances in our knowledge of the physical relations: * 
of the 
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 centréof the instrument has been adapted a simple con- 
‘ trivance for holding two tourmalines, and at the same time for se- 
curing the mica plate in the proper 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 unva-— 
rying 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 
pe requisite number of degrees, the two series of colored rings 
a com 
€ arrangement; the instrument is so adjusted, that the cord 
accurately intersects the black dots of the inner colored circle 
~. about one axis; a revolution is then made, till the cord intersects 
heat. The instrament was so pag a the Locatelli lamp deflected the — 
ted ’ of time 30° of the scale. arrange ed, the following results w 
ained ; 
os wera toes os Needle rercent. 
Mica examined. Optic angle, Color. : anes | rays te 
a ee cael 
Muscovite of Grafton, ...69° 80’ - light brown, -........ 19°-20° |57-60 
“es pite, Po ope’s 5s Mills, 4? nag white glassy, .....s4..| 18°- atiee 86-345 
“ brownish yalow, setvla 15° 45 
oD Edwards, ... 13° 30’ _lyellow brown,........ 15 45 
erie (2) To Me, deep reddish brown, .. 132-1 2° (39-36 
( proba hlogopite. . 
Biotite, witidg ¥ aa dead green alinoit black |] 1° 33 
Muscovite, Royalston, Ms, 57° 30’ dark brown, . y 
weeple oO? nearly colorless clear, . .|21°-21°30’ 63-645 
- ounliewd Me, 72° 30/ light brown,.........- 5 12-21230 63-645 
ey Jones Falls,Md.,)67° dark Qre@hi-is 2s vss ewe 18°=19? Sed 
“Philadelphia, . .|60° 30’-61° casi inhexag’l figures,| 21936'-229|64°5-66 | 
When the crystal was placed so that the dye of heat passed ee to the optic 
axis, (thus the ~arigo mica was placed at an pegs of 34° 30’, the 
remaining otherwise as before) the needle was on repeated trials eflected 24°, 
€qual to 72 per cent. of all the rays a ¢ while i in the other bis "ton with the 
0° pass 
to 
From these few trials (which are regarded as only preliminary and approxima 
tive,) i t will be oor. that — interesting relation stale 2 subsists ott the sort Sechied 
fr, ‘and this last experiment is particularly worthy of confirmation by exte nding it 
arleties 
—~ Blake presented his instrument and a series es of measurements made with it 
the Physical Section of the Am. Assoc., at the New ing sapetion: 
