Double Refraction

chaoseverlasting

Im trying to understand double refraction, but have certain doubts.

One question is, that why does the direction of oscillation of the ordinary and extraordinary ray change their refractive indices?

Why is it that when one component of the electric field is perpendicular to the optic axis, it has a different velocity and the one parallel to the axis has another velocity? I understand that their different velocities lead to their different refractive indices, but dont know why the velocities are different.

Here are some notes : http://edafologia.ugr.es/OptMine/intro/doblerew.htm