For light which travels according to the direction of the optic axis (E-E'), the crystal behaves isotropically. The light which is propagated according to this axis, vibrates in any direction of the plane perpendicular to this axis, with the same velocity, (the structure of the crystal for this plane is very symmetrical, like that presented by the crystals from the Cubic system in any direction) and, moreover, does not present double refraction. The refractive index which corresponds to these directions of vibration is designated w.
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