When
viewed through a dichroscope, two images of the opening are seen at the
opposite end side by side due to the very strong double refraction of
the calcite. The vibrations of the rays producing the two images are at
right angles to each other and correspond to the ordinary and
extraordinary rays. When a pleochroic crystal is placed in front of
the opening, the two images are generally differently coloured. The
colours of the images correspond to the colours transmitted by the two
rays. If a ruby is viewed in this way, one image is dark red while the
other is lighter in colour. In the direction of the optic axis the two
images have the same colour. In a particular direction there will be
maximum difference in colour. In biaxial substances there are three
principal directions at right angles to one another in which the
absorption is different. The dichroscope can reveal two absorption
colours at one time. To observe the three absorption colours, the
specimens must therefore be viewed parallel to two of the principal
directions. When thus examined there will be four images, which will
include the three principal absorption colours and the fourth being a
repetition.
The Petrological Microscope
Pleochroism
could also be determined by the microscope using only the polarizer. On
rotating the pleochroic substance placed on the microscope stage, first
one colour and then the other will be seen at angles of 90°. But in the
dichroscope the colours are seen side by side, and very slight
variations could be easily recognized. Since pleochroism is an
absorption phenomenon, the colours also vary with the thickness of the
specimen. The pleochroism is a property that is also an aid in the
determination of gem-stones; for instance red garnet and spinel, used
to imitate ruby, could be very easily found out because ruby is
dichroic while garnet and spinels are not.
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