Those who have any experience at all in this matter will allow that the rock crystal exerts an influence of an entirely different nature to that observable in the use of glass. Indeed, so far as experiment serves us, it may be said that glass only produces negative results and never at any time induced clairvoyance. If this state followed upon the use of a glass ball I am sure that the patient must have been naturally clairvoyant, in which case a bowl of water, a spot upon a wall, a piece of polished brass or copper, or a spot of ink would have been equally efficacious in inducing the degree of hypnosis required. That glass spheres are equally efficient as those of crystal is true only in two cases, namely, when clairvoyance is natural, in which case neither need be used; and when no results are observable after due experiment, from which we may conclude either that the agent is unsuitable or that the faculty is entirely submerged in that individual.
In hypnotic clairvoyance the glass ball will be found as useful a "field" as the best rock crystal. Yet it does not follow that because the crystal is highly odylic and glass altogether negative the former will induce clairvoyance. My own first experience with the crystal was entirely disappointing, while very striking results followed immediately upon the use of a black concave mirror.
The mirror is usually circular in shape and about one-quarter-inch curve to a six-inch diameter. This gives a long focus, so that the mirror may be hung upon a wall at about two yards distance from the subject. A greater degree of concavity proportionate to the diameter will produce a focus which allows the mirror to be held in the hand while resting in the lap.
This disposes to a very easy and passive attitude and helps towards results. The base of the mirror may be of tin, wood or other material, and it is usually filled with a composition of a bituminous nature, the glass covering being painted with a preparation of coal-tar on its nether or convex side. The exact focus and consequent size of the mirror employed as most suitable to the individual is a matter of experiment. It is also to be observed that the distance of the mirror, as also the angle of vision, are matters of experiment. Beyond a certain distance it will be found that the mirror has no "draw" on the subject. If brought closer its pull is immediately felt.