The Magic Mirror is a block added by Thaumcraft 4. It is used in linked pairs to send items back and forth between one another, even across dimensions. In order to link mirrors, one of them must be placed in the world first and then right-clicked with the other one. Objects can then be thrown into or transported via other means like piping through the mirror. There is a chance that items may not make it through, though this may be remedied by sending CV into it.
You are now able to enchant a pair of magical mirrors that act as doorways to the void between the worlds. Anything shoved (or piped) through one mirror will pass through this realm and emerge from the other mirror - even across dimensional boundaries.
Unfortunately this realm is extremely hostile to mortal creatures like yourself so using these mirrors to transport living creatures is not yet possible.
To link a pair of these magical mirrors you simply need to place one of them in a desired location. You then right-click with a second mirror on the one you placed, and the pair will become linked. You may then place the second wherever you wish and items thrown into one will be ejected out the other.
Mirrors can only be linked in pairs, so while both are placed in the world and able to transmit items you cannot link them to another mirror. When you pick up a mirror it will remember the last partner it was linked with, allowing you to easily move it without having to relink it. While one of the paired mirrors is picked up, you may easily relink either of them.
Transporting items through mirrors places stress on the magical glass and to prevent it from shattering the mirror automatically slows down the rate at which items is spit out the other side. The greater the instability, the more the transfer rate is slowed.
This instability slowly lessens over time, but it can also be negated by relaying Ordo vis directly to the mirror.