This is a glass slide of 'automatic writing', spirit writing made by a medium. Automatic writing is alleged to be created by spirits taking control of the medium's hand who remains largely unconscious while the message is being transcribed.
The message on this glass slide appears to read:
Friend Phillips –
You may say to your readers that our medium is here to demonstrate this phenomena and that the good work will go on, in a quiet and convincing manner – signed Guides – XXXXX XXXXX
Then upside down it reads:
I am happy and glad to come to you now. XXXX up in your work and look for better XXXX. XXXX will be victorious. The two worlds will be the great help.