**Determination
of the birefringence of a mineral**

The interference colour is the result of a retardation between the waves which are vibrating inside the mineral and is defined by the equation:

R= e(n1 - n2)

where "R" represents the retardation, "e" is the thickness of the mineral, and "n1" and "n2" are the refractive indices of the waves.

This equation has been solved graphically in the chromatic scale of Michel-Levy.

This is an extremely useful chart for calculating the birefringence of the minerals from their interference colour.

It is very simple to use. Moving along the horizontal line which corresponds to the thickness of the microcscope preparation (normally between 20µm and 30µm), the band of interference colour presented by the mineral is located, and then moving up the sloping line, which passes through this colour, we will obtain the birefringence value of the mineral.

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