The Dctcnnination of Feldspars. — Spiirr. yjj 
and make the same observation. One such contact generally 
gives results reducing the feldspars to very narrow limits, and 
often determines the species exactly. 
e. g. In one position one determines that in the feldspar 
"'g>-np of the quartz, . • . feldspar "'g > I-544- 
In a position at 90° to the first' one finds «'p > "g i. e. 
"'p > I-553. 
Referring to tables showing indices of refraction for various 
minerals, this fixes the feldspar as labradorite. 
In selecting minerals for comparison, the selection of feld- 
spar should be made with reference to a known section, such 
as a bisectrix. Any adjacent quartz grain may be selected, 
whether or not its chief axes of elasticity lie parallel with those 
of the feldspar grain. The value of those directions which 
do lie parallel to the chief axes of elasticity of the feldspar can 
easily be reckoned. 
Method for Deteniiining feldspar and other Minerals through 
their double refraction by means of Michel Levy's 
Coniparateur* 
First correct the scale of the comparateur, which can not be 
accurately made, so that o of the scale corresponds exactly to 
the thickness o of the quartz wedge of the comparateur. Do 
this with the teinte sensible plate. Insert the plate in the mi- 
croscope below, and by shoving along the quartz wedge of the 
comparateur equalize successively the teintes sensibles of the 
first and second orders on the wedge, and read on the scale. 
Let these, two readings be 375 and 720, symbolized by t and t' 
In thin sections dispersion may be neglected. It is known that 
t (375 in this case) corresponds to a retardation of the light 
waves of 0.000575mm'. and t' (in this case 720) to one of 
0.001128mm. Therefore the division of the scale taken as 
unit in the measurement (in above case the smallest division 
1128—575 
of the vernier, s}-mbolized by d) equals d = -, — ^^^-^ in 
millionths of millimeter. In the above case d= 
t'— t 
1128—575 
720—375 
1.6087 i" millionths of millimeter, or 0.0000016 millimeter re- 
tardation of light wave in the thickness of the quartz wedge, 
• Michel Levy Mineraux des Roches, p. 34, etc. 
