12 
SORBY ! OPTICAL CHARACTERS OF MINERALS. 
microscope may then be employed along with afor i object- 
glass, and, if properly constructed and used, the requisite 
measurements may be made to within 20( ^ 00 th of an inch. 
We may thus approximately determine the indices in sections 
only jotto" of an inch in thickness. It is, however, necessary 
to adopt a system which reduces the number of separate 
measurements and to a great extent eliminates several sources 
of error. Instead of attempting to measure the absolute 
thickness of any particular crystal, and the actual displace- 
ment of the focal length due to it, the apparent thickness of 
the mineral, as seen through itself (t), is measured by means of 
the rotation of the graduated circular head of the fine adjust- 
ment by focussing, first to the top and then to the bottom, of 
some appropriate specimen. In each particular substance 
this apparent thickness is equal to the true thickness divided 
by the index of refraction. The thin glass cover is made 
somewhat larger than the section, so as to project beyond it, 
and inclose a layer of the hard and brittle balsam used to 
fasten down the piece of rock. Selecting for observation a 
specimen as near as possible to this balsam, so as to avoid any 
error due to unequal thickness, the difference (d') f in the dis- 
placement of the focal length due to the mineral, and the 
balsam is ascertained by focussing through each the lines of 
the grating. This value is positive or negative, according as 
the index of the mineral is greater or less than that of the 
balsam. It then follows that t±d' is the thickness, as seen 
through itself, of the amount of balsam of the same real 
thickness as that of the mineral : the effects of the balsam 
below and above it, and of the covering- glass, being thus 
entirely eliminated. If, then, the index of the balsam be m, 
we can easily calculate that of the mineral (/*) from the 
following equation : — 
t±d 
