I have rarely observed the existence of isolated auditory hallucinations; I have always observed them associated with visual hallucinations; therefore I will study them after these last, when examining mixed phenomena.
(c) Visual sensations.—Observable, visual phenomena are very numerous, and have already been the object of exhaustive studies. I will again divide these into provoked and spontaneous phenomena. Of course, I am speaking of hallucinations experienced by sensitives out of seance hours. In this part of my analysis, I am replacing the word medium by the word sensitive, which seems to me to define more correctly the distinguishing features, of those persons who have the faculties I am going to describe. This word conveys the correct idea, that the facts observed belong to the sphere of sensibility.
One of the oldest known methods of inducing visual hallucination is the use of a crystal ball. I have no need to recall to mind the practices of former fortune-tellers, nor the history of John Dee, nor the numerous recitals handed down to us by ancient chroniclers, novelists, etc. The crystal ball and the black mirror are the best methods; but the ordinary mirror, a glass of water, a decanter, a shoemaker’s wooden ball, the finger-nail, the watch-glass, any polished surface, in fact, may serve to induce hallucination; but I only recommend the first methods—they are certainly the best; a glass of water, a decanter, a syphon of seltzer-water, the thumb-nail, polished surfaces, etc., may serve to induce hallucination, but these last methods only succeed with very highly sensitive subjects.
I have carefully studied crystal-gazing, and though I have remarked individual differences in each sensitive, I think I may say that, as far as working methods are concerned, I have come to the following conclusions:—
The material of which the object is composed is not a matter of indifference. Balls of rock-crystal have given me the best results. I have seen people, incapable of receiving visions with ordinary glass, obtain them in a tiny ball of natural crystal. Objects in rock-crystal have the inconvenience of being very expensive.
Ordinary glass gives good results, but care should be taken that the ball contains no air bubbles or other defects. They must be as homogeneous as possible.
The ball may be spherical or egg-shaped. I think the elliptical form is, perhaps, the best; reflections are more easily avoided with this shape.
The size is a matter of indifference; personally, I prefer rather large balls. I have, nevertheless, obtained just as good results with balls of only one centimetre in diameter as with balls of six or seven centimetres in diameter.
The crystal may be white, blue, violet, yellow, green; it may be opalescent or transparent; but, I think, the best results are obtained with white transparent balls; blue or amethyst coloured crystals are also very good, and tire the eyes less than others.[8]