Quartz crystals in a range of liquids from n= 1.528 to
Figure 1: liquid index matches ordinary index of quartz.
Central crystal has ordinary index parallel to polarizer and appears thus
with a red-orange right side and a blue left side. Only the crystals near
the border of the dark side can be viewed with this technique.
Figure 2: same index of liquid but rotation of the stage by
90°. The central crystal has now extraordinary index (higher) parallel to
polarizer: the right side becomes bright orange.
Figure 3: the lowest liquid refractive index: all crystals
have dark blue and pale yellow sides.
Figure 4: liquid index slightly lower than ordinary quartz
index: all crystals on the dark border have an orange to yellow right side.
Figure 5: the highest liquid refractive index (equal to the
high index of quartz): the central crystal oriented nearly parallel to the optic
axis has a dark red side (nearly matching for the D line of sodium), other crystals in
random orientation appear with a dark right side (brown to black) due to their low
index of refraction and a blue left side.
Note that the colored
effects are better seen with phase contrast on the whole field of view but the
cost of oblique illumination is of course much lower. Phase contrast cannot be
used if the indices of crystal and liquid are too far from each other because
the phase difference can easily reach more than 360° and thus give a wrong
interpretation. In that case, for the first examination of a crystal, I always
use oblique illumination.