Note 11 — Chapter III · ASTRAL LIGHT AND THE PSYCHO-LASTROMETER

The psycho-lastrometer was further perfected. The element selenium is inordinately sensitive to all forms of light rays and according to the light thrown upon it permits more or less electric current to pass. I arranged the apparatus so that the light thrown out by the psycho-lastrometer impinged upon a selenium cell whose resistance varied from 50,000 ohms to 100,000 ohms, which was in its turn connected to a cell and to a Deprex d’Arsonval mirror galvanometer. This enables accurate readings of the actual waxings and wanings of the light value of the lastrometer plate to be taken, and entirely eliminates any possibility of visual illusion seeming to make the plate more luminous than before. A series of plotted curves based on time abscissæ and light co-ordinates will give an accurate scientific record of any differences in the radiant value of the plate that occur during the séance.