Psychomanteum is a room, that is small, and very dark, with only a candle burning, and a mirror which is angled so it does not reflect anything, and this room was invented be Dr. Moody to connect with spirits.
“The Psychomanteum is a chamber invented by Dr. Moody, but inspired by the ancient techniques used for 2500 years at the Oracle of the Dead in Ephyra Greece. A visitor to the Psychomanteum often experiences contact with departed loved ones.” http://www.lifeafterlife.com
Psychomanteum came from Ancient Greece where people would stare into anything with a reflexation that would produce visions. So the Psychomanteum room is designed to connect with the spirit world, so it has to be dark, and with a mirror so you can see the spirit world through the mirror.