2S 
The American (kolo^ist. 
January, 189>< 
In convergent light with a high power (Nachet No. 7 ob- 
jective), bring the focus directly upon the Hne of separation 
between a quartz grain and a feldspar. On lowering the con- 
denser and removing the analyzing nicol the field is a little 
darkened, but a very fine line of white light, clearer and 
brighter than the grain on either side, accompanies sharply 
the line marking the contact of the two grains. When the 
objective is focused so that the line is bright, if the objective 
be raised very gently, and the least amount possible, this 
bright line moves a little toward the more refractive mineral 
before it is extinguished. If the objective be lowered in the 
same way the white border line shifts a little toward the less 
refractive mineral. This method is most useful for distin- 
guishing between orthoclase and quartz and between the fresh 
secondary plagioclases of the crystalline schists and quartz. 
The other feldspars are usually distinguishable by other char- 
acters. It is to be employed with one condition, viz: when 
two adjacent minerals of nearly the same refractive index 
happen to be cut, one perpendicular to n^ and the other to n^ , 
the movement of the line might be governed by the difference 
between n^ and n^ of the minerals, rather than by the differ- 
ence of their mean refractive indices. 
The table below shows the indices of refraction and the 
double refraction of the feldspars as given by Levy and La- 
croix (Min. des Roches, p. 323): 
Orthoclase . . 
Microcline . . 
Anorthoclase 
Albite 
Oligoclase . . , 
Andesine . . . 
Labradorite . 
Anorthite , . . 
Ouartz 
Refraction. 
1.526 
1.529 
1-530 
1.540 
1.542 
1.556 
1.562 
1-553 
-523 
.526 
-529 
-534 
•538 
-553 
-557 
.566 
519 
523 
523 
532 
534 
549 
554 
Double 
Refraction. 
1-544 
o . 007 
0.006 
0.007 
0.008 
0.008 
0.007 
0.008 
0.013 
0.009 
