It is formed of astral or terrestrial light, and transmits the double magnetization of it to the human body. The soul, by acting on this light through its volitions, can dissolve it or coagulate it, project it or withdraw it. It is the mirror of the imagination and of dreams. It reacts upon the nervous system, and thus produces the movements of the body. This light can dilate itself indefinitely, and communicate its reflections at considerable distances ; it magnetizes the bodies submitted to the action of man, and can, by concentrating itself, again draw them to him. It can take all the forms evoked by thought, and, in the transitory coagulations of its radiant particles, appear to the eyes ; it can even offer a sort of resistance to the touch. But these manifestations and uses of the medium being abnormal, the luminous instrument of precision cannot produce them without being strained, and there is danger of either habitual hallucination, or of insanity.
By augmenting the elasticity of the vital light and its force of projection, one sends it forth as far as one will, and withdraws it completely loaded with images ; but this operation must be favoured by the slumber of the subject, which one produces by coagulating still further the fixed part of his medium.
—Eliphas Levi from “The Key of the Mysteries (La Clef des Grands Mystéres)” , translated by Aleister Crowley, as found in The Equinox, Vol. 1, No. 10.