Major-General McMahon—Double- Refraction of Minerals. 551 
When a mineral contained in one slice has to be compared with 
a mineral in another slice, we must, if we desire to be exact, measure 
the thickness of our slices, and take the disturbing element of the 
difference of their thickness into consideration. But for the rough 
and ready mode of estimating double-refraction now under descrip- 
ation I have not found this necessary. Rock-sections prepared by 
a skilful lapidary—especially if we compare slices of rocks of the 
same class'—do not differ greatly in thickness; and if we always 
employ the same lapidary, we shall not go far wrong if we assume 
that our slices are, for the purpose in hand, of fairly equal thickness. 
The difficulty connected with the fact that slices of rocks for 
microscopic study are made promiscuously without reference to the 
orientation of individual minerals, and that the sections of the latter 
are consequenly cut at varying angles to their optic axes, may to 
a great extent be overcome by selecting for examination those 
erystals that display the brightest colours under crossed Nicols; for 
the sections that polarize brilliantly are presumably those made 
approximately parallel to an optic axis. 
Notwithstanding the difficulties alluded to above, the quartz 
wedge is a delicate, as well as handy instrument, of great value 
in the practical determination of the minerals met with in rock- 
sections. It enables one, for instance, to separate at a glance such 
minerals as rutile, dolomite, calcite, sphene, anatase and zircon, from all 
others likely to be met with. It will enable us to detect even a speck 
of granular calcite of microscopic size imbedded in another mineral. 
The double-refraction of the above minerals is so powerful that 
when they are tested in the way above described, the dark line* does 
not come into view until the thick end of the wedge begins to pass 
over them, and the spectra, instead of being of normal width and 
distance apart, are narrow and crowded together. The character of 
these narrow and crowded spectra is so remarkable that an observer 
who has once seen them can never mistake them for those of a 
mineral possessing a less intense double-refraction. 
So powerful is the double-refraction of rutile, calcite, and sphene, 
that I sometimes find it necessary to employ two quartz wedges, one 
above the other, in order to bring the spectra within the range of 
vision. Occasionally the mass of a crystal resists even the double 
wedge, and the spectra can only be seen in portions of the crystal, 
where, owing to its brittleness and liability to fray in the process of 
grinding, or from its edges overlapping other minerals, specially thin 
sections are exposed to view. 
Minerals of feeble double-refraction, on the other hand, may be 
separated from those of strong, or of even average double-refraction, 
with equal confidence. If we examine such minerals as chlorite, 
nepheline, and apatite, we find that the dark line, described above, 
either appears on the very edge of the quartz wedge, or is just 
1 Basalts, for example, are usually sliced thinner than granites. 
2 When testing for double-refraction the stage must be revolved until the dark line 
appears in the mineral under examination, The azimuth in which this appears affords 
another valuable means of distinguishing between different species, 
