A minor addition to clarify: if one did the same experiment inside an ideal gas, rather than a crystal, there are still atoms in the way (compared to vacuum) but there is no preferred direction through the atoms. Crystalline structure gives a preferred direction along the principal axes. This might mean there is less optical extinction along the principal axes from deep inside the crystal, for example, whereas from deep inside the ideal gas optical extinction is the same in every direction.

That is, the view varies depending on direction one looks within a crystal. "Symmetrical" means that the view should not vary that way. If we freeze our gas into a crystal lattice, we break this symmetry.