The explanation is as follows: I possess, let us say, 100 units of psychic energy which I might use in attention.
The phenomena are not caused by the medium, but by psychic energy acting through a medium, and that is a very different thing.
For a moment I gathered all my psychic energy and concentrated on the single crucial problem of stopping that leak.
How the crystal, magic-mirror, etc., serves to focus the psychic energy of the clairvoyant person.
psychic energy is indeed a fastidious thing, that insists upon having its own conditions fulfilled.