Here is French's personal method of making a "spirit answer" appear on a borrowed slate ... while sealed inside an envelope and held by a spectator. The spectator's initials may be written on the frame of the slate to prove that there are no switches of any kind employed. This mind-numbing mystery can be worked solo, as a one-man effect, with an ungimmicked slate. Instead of a slate, black cardboard or artist board can be substituted. Complete routine, with three presentations described, for stage, party or séance. Absolutely no flaps, switches, gimmicked slates or chemicals used. No stooges or forces, either.
The original manuscript was quite expensive in its day, selling for the equivalent of $35 for the secret alone. This title is nearly impossible to find nowadays, being jealously hoarded by collectors.
If you've wanted to perform a test conditions spirit slate message effect, this routine is one you must add to your working library.