So far as I can tell, no human mental effort conscious or subconscious can affect this simple instrument. It is necessary to guard against illusion by imagining that the lastrometer is gaining radiance, and to this end it is advisable to prepare a stand and test-piece made of aluminum and coated with precisely the same solution as is applied to the plate. These should always be kept together and allowed to become equally radiant. If this is placed on the table near the lastrometer, any variations in the latter can be rapidly verified by comparison with the non-insulated and non-oriented test-piece. Antipathy on the part of the presiding medium to the use of the psycho-lastrometer is invariably a bad sign. Spirit messages objecting to it are the most valid reasons for its retention, and such communications should be viewed with the deepest suspicion. The cost of the apparatus is a few shillings, it can be made by anybody in an hour or so of spare time, and in actual point of fact there is nothing about it that is offensive to the spirits of “good” or to the pure. To those who are learned in symbolism I may suggest that the receiving disc at the top of the wire need not be in the form of a disc, but can be cut or pierced with ornament such as sacred symbols or with any decorative design. It is desirable, however, that the light surface be retained and that the available metallic surface of the disc should not be diminished more than is necessary. FOOTNOTES: [8] _Notes of an Enquiry into the Phenomena Called Spiritual._ William Crookes, F.R.S., p. 91; Class VIII: Luminous Appearances. [9] For details concerning ectoplasm see _Ghosts in Solid Form_, Gambier Bolton, etc. [10] Something of this view may be found in the chapter on “Pseudo Spirit Phenomena” in _Borderland of Psychical Research_, H. J. Hyslop. A book deserving of attention by all interested in Spiritualism. [11] 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. CHAPTER IV AN EXPERIMENT ON THE THEORY OF PROTECTIVE VIBRATION