Hi Anita,
Birefringence is exhibited by a substance, often crystalline, whose
molecular structure is aymetrical so that two rays of light in perpendicular
planes will travel at different velocities through the substance, producing
a fast ray and a slow ray. This substance has two refractive indicies and is
said to show positive birefringence if the plane of vibration of the slow
ray is parallel to the length of the crystal, or negative if the plane of
vibration of the slow ray is perpendicular to the length of the fibre.
Normally birefringent substances appear colourless (white) against a dark
background under crossed polars, but where the thickness of the crystal of
fibre is uniform, interference colours may be produced when two rays (fast
and slow) are reunited in the analyser.
Dichroism is due to light-absorbing differences along different planes of an
asymetrical substance. The spatial arrangement of light-absorbing bands
(resonating electrons) is such that either light of a definite wavelenght is
selectively absorbed or a change in intensity of white light occurs when
light passes through the substance in certain planes. The dichroism shown by
amyloid  is a property of the dye molecule, e.g. Congo red or toluidine
blue, whose linear molecules are bound by hydrogen bonds along the parallel
folds in the beta-pleated sheet protein, producing different light-absorbing
characteristics along certain planes of the fibril. Dichroism is studied in
tissue sections using one polariser only.
Hope this helps,

Evelyn Kaplan
Sultan Qaboos University
Oman