It should be pointed out that this book does not attempt to deal with abnormal phenomena which occur spontaneously—with apparitions or forms resembling the dead coming unsought for, such as have been recorded in all ages of the world's history. It will be seen that such phenomena have nothing in common and cannot be said to be identical with those which are invoked and induced, for which a circle has to be formed, for which a medium is employed, and for which favorable conditions have to be created. It is with such phenomena alone that this book deals.