84
Adjust the collector (48.19) to an optimal setting,
using a diffusing screen (42.15) if necessary.
Determination of optical character
Uniaxial crystals (Fig. 59a)
Uniaxial crystals observed in the conoscopic
(divergent) beam show a dark cross, whose
centre indicates the position of the optical axis.
The cross is surrounded by coloured inter-
ference fringes*. When a variable compensator
(quartz wedge or tilting compensator) is oper-
ated the rings drift towards the centre or out-
wards in two opposite quadrants of the cross.
The optical character is determined from the
direction of movement of the rings as in Fig. 59.
Cutting directions in which the optical axis of
the crystal is inclined to the direction of
observation are suitable for the determination of
the optical character, which can mostly be
determined even when the centre of the cross is
outside the field of view. Fig. 59 shows that fixed
instead of variable compensators can also be
used for the determination of the optical
character.
The optical character can usually be identified
even when only one of the optical axes is in the
viewing direction of the observer. In the
orthoscopic beam the brightness of specimens
orientated in this way changes little if at all
during rotation. In the conoscopic beam, only
one of the two isogyres will then be visible.
Biaxial crystals (Fig. 59b)
For the determination of the optical character
cutting directions are particularly suitable in
which the bisectrix of the two optical axes is
parallel to the viewing direction (section vertical
to the acute bisectrix).
In the divergent beam a dark cross will be seen
which opens up into the two branches of a
hyperbola, the so-called isogyres, when the
stage is being rotated. The cross and the
branches of the hyperbola are surrounded by
interference fringes. According to Fig. 59 or the
rule mentioned below the optical character can
be determined from the displacement direction
of these fringes after operation of the com-
pensator. The symmetry plane of the isogyres
(axial plane) must be vertical to the
γ
direction of
the compensator.
* Only the cross is visible for thin specimens or
specimens with low birefringence.
