552 Major-General McMahon— Double- Refraction of Minerals. 
beyond the range of vision. In the latter case I call in the aid of 
the + undulation plate: on inserting this above the object-glass, the 
spectra seen in the quartz wedge are shifted up the wedge towards 
its thick end, and the dark line previously beyond the range of 
vision is brought on to the edge of the wedge. 
In the case of the above-mentioned minerals possessing very feeble 
double-refraction, the dark line appears, as just stated, at the very 
edge of the quartz wedge. In the case of sanidine, orthoclase, and 
other minerals of equally weak! double-refraction, the dark line 
appears clear of the edge of the wedge, but no chromatic band comes 
in between it and the edge. As we examine minerals of higher and 
higher double-refraction, we shall find the position of the dark line 
is more and more shifted towards the thick end of the wedge, and 
more and more chromatic bands come in between it and the thin end 
of the wedge. 
Thus, to give a few examples drawn from my own experience of 
slices prepared by different lapidaries which are not all of equal 
thickness, I find that quartz in such sections rarely exhibits more 
than one and never more than two chromatic bands between the 
dark line and the edge of the wedge; whilst such minerals as 
muscovite, olivine, and actinolite, commonly present three, and not 
unfrequently as many as five such bands. 
We have here, then, a means not only of comparing sections of 
minerals of equal thickness in the same slide, but sections of crystals 
in different slides, and we have a standard by which we may estimate 
the strength of their double-refraction. A mineral of strong double- 
refraction sliced at so high an angle to an optic axis as ta approximate 
a plane normal to that axis will, of course, exhibit little or no double- 
refraction, and in the phenomena it presents will resemble a mineral 
of feeble double-refraction sliced approximately parallel to an optic 
axis; but a mineral of weak double-refraction can never exhibit the 
phenomena characteristic of one of strong double-refraction. When 
therefore. on testing a mineral with the quartz wedge, we find two, 
three, or five, chromatic bands appear between the dark line and the 
edge of the wedge, we know for certain that the mineral possesses 
stronger double-refraction than those minerals which never, in slices 
of normal thickness, present to our view more than one such 
chromatic band. 
Even in the case of minerals of strong double-refraction sliced 
approximately normal to an optic axis, we gain something by 
examining them with the quartz wedge, for we learn from the 
position of the dark line? that the section is one either approxi- 
mately normal to an optic axis, or is one of a mineral of feeble 
double refraction. If the former is the case, the mineral will exhibit 
characteristic appearances when examined in converging polarized 
light. If, on the contrary, it does not show any phenomena capable 
1 T use the word weak to indicate a slightly higher double-refraction than feeble. 
Thus, y—a for chlorite is 0001, but for sanidine is 0:008. 
2 The dark line will not appear in any azimuth in a section guite normal to an optic 
axis. 
