“The Trap” (1931)
Read in 2018.
A teacher loses one of his students, a 15-year-old boy, in a mirror.
The focus is on the premises of the mirror world. This is unfortunate since it is not actually clear how self-imposed exile therein is supposed to help the Danish sorcerer Axel Holm, nor why food becomes unnecessary, why colours are inverted etc. It is particularly mysterious why Holm would bring slaves. He must want to live forever with a couple of guys who have good reason to hate and overthrow him. The psychic communication through dream seems a poor choice of mechanism to convey all of this unused and inelegant worldbuilding.