In his experiments in crystal-gazing, Dr. Dee evidently used more than one crystal, and did not indeed confine the operations of his scryer or scryers to brilliant spheres. In the collection of Horace Walpole, at Strawberry Hill, was a polished slab of black stone, obsidian, from Mexico. This came into the possession of Mr. Smythe Piggott and later (1853) into that of Lord Londesborough; it is now in the collection of Prince Alexis Soltykoff. Horace Walpole wrote a label for the stone, in which he says that it had long been owned by the Mordaunts, Earls of Peterborough, and was described in the catalogue of their collection as the black stone into which Dr. Dee used to call his spirits. Later it was owned by John Campbell, Duke of Argyle, who gave it to Horace Walpole.[294] Undoubtedly any polished surface, whether flat or convex, might serve the purpose of the scryer almost equally well; the possible advantage of a convex or a spherical form consists in the multiplying of the reflections and light points so that the sight is induced to wander from point to point, and that forms and even motions are suggested by the superposition and combination of the various reflections. Often, too, a light point visible to one eye will not be so to the other, this sometimes provoking the phenomenon of binocular vision, which asserts itself for a moment or two, when the diverse images coalesce again, though imperfectly, giving an impression of movement. For one gifted with imagination and the natural quality of visualizing brain-pictures, these shifting light-points and the more or less definite and repeated reflections of surrounding objects offer abundant material out of which to construct lifelike pictures apparently seen in the crystal. That the brain-pictures thus thrown out, so to speak, upon the crystal, may or may not have a peculiar psychic value, other than their value as mere phenomena, depends upon the significance we are inclined to attribute to the processes of the subconscious intelligence; of its existence, indeed, there can be no doubt, and many of our best thinkers incline to the belief that through it the narrow limits of our personality are occasionally transcended. [Illustration: 1, 2, 3. Rock-crystal spheres having portions of the surface ground so that they are rendered partially opaque. 4. Natural cross of rock-crystal. On dolomite, Ossining, New York.] The following history and description of a crystal ball is given by John Aubrey (1626-1697):