How to place a mirror parallel to a wall? For one of my experimental setup I need to place a mirror perfectly parallel to a wall. It can be placed at any distance from the wall. I would like to use any method other than direct measurement. I am free to use the following:
a webcam
a secondry mirror
Edit: It's not necessary to use all or either of them.
 A: Forget the webcam.
attach the secondary mirror to the wall, at a height that is near the height of the center of the primary mirror.
Then adjust the horizontal and vertical tilt of the primary mirror to center the multiple images of the secondary mirror within each other on the primary mirror.
If you have a cheap laser pointer and a carpenter's square, you can set up the laser pointer so that it is exactly square to the wall (vertically and horizontally) and then adjust the primary mirror such that the laser beam goes back to the laser.
A: Fasten the secondary mirror to the wall, facing out, and support the webcam centered in front of it, looking into it. Turn on the camera. Rotate the webcam up and down, left and right, until it's image of itself in the secondary mirror is centered. Put the other mirror behind the webcam: you will see what looks like a great many images of the camera if the mirrors are not parallel to each other. Try it: you should see right away what you have to do to make the mirrors parallel to each other (and to the wall). 
You said the mirror can be any distance to the wall, so an alternative method is to set that distance to zero.  
