In other cases they take the form of direct rays and in certain individuals have been known to occur as flashes like dull electric discharges. Another not uncommon form is the projection from the body of a distinctly defined aura or radiation of light which is faintly luminous like the gases in a Geissler tube subjected to oscillant discharges. We must go far back into history and indeed beyond the bounds of history before we can come to a time when this manifestation of light was not one part of the common stock in trade of the thaumaturge or wonder worker. The manipulation and control of astral light phenomena were part of the religious mysteries of the magicians of Chaldea who transmitted the secret knowledge to the seers of Egypt. We find it in the myth of the luminous bull in the Greek mysteries and again as an attribute of the great healer Apollonius of Tyana. This mysterious radiance plays equal parts in the records of the lives of the saints and in the terrible archives of the trials for sorcery. Confusion exists because to the untrained eye of mankind all forms of astral light are identical. The greater proportion of the astral light seen by circles is that generated and given off by the human mental energy of the circle itself. The spirit-forms are all too often thought-forms built up out of the liberated psychoplasm or thought-matter given off by sitters. The physical nature of this psychoplasm has so far defied all attempts at scientific research, but it appears to be something more substantial than the mere emission of vibrations that it is commonly held to be. It appears to be an all-penetrating imponderable emanation which dissipates rapidly, but which under certain conditions is capable of being energized by the intelligence of the living or by discarnate intelligence. Under these conditions it becomes luminous and under certain further conditions can be used as the vehicle for the transmission of force. It can best be realized as being to the mind what ectoplasm[9] is to the body of the medium, but the precise limitations of both the astral body-matter ectoplasm, and astral mind-matter psychoplasm are not yet ascertained. It is a conceivable hypothesis that both are functions of the vast unknown mechanism of the subconscious self, but where the capacity for the projection of ectoplasm is rare, the emission of psychoplasm is the basis of most Spiritualist phenomena.