You examine the mirror closely, and consider touching it with your fingers, but remember H's warning about skulls and fingers.
Lacking any useful tools in place of your fingers, you consider your limited options.

You could take the pencil nub from the previous "mirror" and examine this one with it - but that would mean H couldn't write back, and your fingers wouldn't be far from its tip anyway. Not an option, especially if H doesn't show up in the attic, and it gets destroyed somehow.
Backtracking to the basement's out: you can still hear angry emanations from behind the door.
Maybe throwing some footwear at the skull "reflection" will solve your problems.
Of course, you could ignore H's warning, and risk touching it with your bare hand, or smashing the mirror to bits with your fists.