Allow me to rewrite above thought experiment (dated on 14 Oct) please.
There is a pillar made of glass (laid horizontally). A light ray (emitted from a light source that is fixed at the upper face of the glass) is propagated through the glass like letter V (reflected by a mirror : 30 degrees : from the left to the right). In front of an observer (at a stand-still), one pillar is moving to the right, another pillar is moving to the left at the same speed. To an observer, whether speed of light in the glass is different or not ? [Postscript] Galileo transformation seems to stand up. [Postscript] If another light ray is propagated from the right to the left (on the same path) ?