dispensed
with, and the instrument used for various other purposes of angular
measurement. Many alterations have been suggested for the purpose of
insuring greater accuracy, but the simple instrument is sufficient for
all purposes of determinative mineralogy, and the error from the
instrument will in most cases be less than the actual variations in the
dimensions of the crystals. Greater simplicity is indeed rather
desirable, and the student will often find it sufficient to attach the
crystal by a piece of wax to the axis a directly, and give it the further adjustment by the hand. The only use of the parts from b to q is
to enable the observer to place the crystal properly; that is, with the
edge to be measured parallel to the axis of the instrument, and as
nearly as possible coinciding with its centre. This is effected when
the reflection of the horizontal bar in the two faces appears parallel
to that edge.
Macles or Twin Crystals.
When
two similar crystals of a mineral species are united with their similar
faces and axes parallel, the one forms merely a continuation or
enlargement of the other, and every crystal may be regarded as thus
built up of a number of smaller crystals. Frequently, however,
crystals are united according to precise laws, though all their similar
faces and axes are not parallel, and then are named macles or twin
crystals. In one class of macles the axes of the two crystals are
parallel, and in another they are inclined. The former only occur among
hemihedric forms, and the two crystals are then combined in the exact
position in which they would be derived from or reproduce the primary
holohedric form. The second class, with oblique axes, occur both in
holohedric and hemihedric forms, and the two individuals are placed in
perfect symmetry to each
