"Rudimentary mirror communication," Strange explains, as he still kind of holds the mirror out, waiting for Childermass to take it. "It's not as advanced as the radios, it's limited to the size of the mirror itself, and the sound quality is terrible, but it does provide visuals."

If Childermass takes the mirror, he'll see that the mirror's reflection isn't it's own: this mirror reflects what's shown in the second mirror, the one that Strange is holding. Strange is still impatient as all hell, waiting for Childermass to take the mirror and examine the magic.

"I think I can make one more mirror to connect with these two, but that would limit it's use to magic users only—you'd have to use your magic to swap between the different reflections."