The Body of Light is the part of consciousness that leaves the corporeal body and carries your senses and consciousness during astral travels.
As to the substance of the body of light, it may be assumed that this is more or less equivalent to the subtle electrical current called by some Prana, Chi, etc., which is formed into a usable form in the likeness of the practitioner. It is thus at the same time an integral part of the self, and also something commanded by the self, much like a puppet or marionette doll.
The body of light, or 'scin laeca', thus 'works' in a realm far different from the normal 'physical' world which is seen as the interaction between the various elements, most noteably the earthen elements, with this energy. It is very likely probable that the body of light is a subtle manipulation on the part of the magician of the very electrical current known to be the stem cause of consciousness and life in humans and other living beings, and it can perhaps be further said that it is the 'active' and 'living' electrical current in all matter which when given form exists as varying spirits.
Thus the 'Astral Realm' is seen as much different from the physical world, due to the very greatly varying 'amounts' and frequencies of this subtle force within the various matters of the physical world. (Thelmapedia)