378 The American Geologist. J""^- i^^^- 
corresponding to the movement of one division of the vernier. 
Supposing the wedge thinned to o, and calHng this reading 
T, we deduce the formula T = "" ^ ^J'^ . In the above 
I 125 — 575 
case T = 16.27; this is therefore the correction, and to be de- 
ducted from the readings; to obtain their actual value. 
A given reading on the scale may be called t". Then e"X= 
(t" — T)d, when e symbolizes the thickness of the mineral and 
X the index of double refraction {n^—tip or m — £). 
It is evident that to use this method directly one must know 
e to find X, or vice versa. The direct methods of measuring e 
are inaccurate, therefore in practice a method of comparison 
with a known e is used. In the same section with the mineral 
to be determined, a known mineral is selected, preferably quartz 
parallel to the vertical axis. It is known that in quartz X (ng 
— „p ^ w — c) —.00915. The interference colors of the 
quartz and of the mineral to be determined are read, and cor- 
rected by T. Then for the mineral studied eX = A ; for the 
A A 
quartz eX' = A'. From this follows X = ^ X' or X = -a"- 
(.00915)). 
If the colors are too high or too low to be distinct, use a 
teinte sensible quartz plate ; if the colors are too low, add the 
teinte sensible (like axes parallel) ; if too low, subtract (like 
axes crossed), then add to or substract from the reading, the 
value of the teinte sensible on the scale, as previously determined 
and corrected by T. For exa:mple, the teinte sensible alone reads 
370. This minus T(— 16) = 354. If a feldspar or other min- 
eral with low refraction colors is to be measured, add the teinte 
sensible, and read the graduation on the scale, which gradu- 
ation corresponds to the sum of the colors. Say e.g., this is 
590: correct by T. 590 — Ti=574. Subtract the value of the 
teinte sensible, 574 — 354=220. Above formula then becomes 
220 
X= — r-,- .00915. For too high colors subtract the teinte sen- 
sible. 
