You try carefully touching it with your penknife, but it goes past the frame, through the threshold, poking the skull.
The skull recedes and disappears from sight, leaving you with a way out of the room, into what appears to be a yard. Deciding it better to escape the room before something weird happens, you enter the "mirror."

A small warped fence blocks your path forward, though you could jump over it into the fields yonder.
You can also choose to walk left or right.
You look back, but all you see and feel is a solid stone wall with a drawing of a skull on it.