Door Simple

I was in an art gallery many years ago,
with an 'impossible' door.

The back wall was all of mirrors,
so constructed that a person
approaching them would seem
to disappear to anyone across the room.
Upon investigation one would find a
door set at right angles to the wall,
and not visible from the front.
Yet this door would not open.
Dismay??
This was a ruse --
for if you simply walked straight back
'into' the gleaming central mirrors
you would be in a hallway.

So, everywhere you looked
you only saw where you had been,
but could proceed on faith alone.