The explanation of this mysterious phenomenon has been but recently arrived at. The discoveries of Von Reichenbach, of which I gave a sketch in the first letter, announce the principle on which it depends. Among these discoveries is the fact that the Od force makes itself visible as a dim light or waving flame to highly sensitive subjects. Such persons, in the dark, see flames issuing from the poles of magnets and crystals. Von Reichenbach eventually discovered that the Od force is distributed universally, although in varying quantities. But among the causes which excite its evolution, one of the most active is chemical decomposition. Then, happening to remember Pfeffel’s ghost story, it occurred to Von Reichenbach that what Billing had seen was possibly Od light. To test the soundness of this conjecture, Miss Reichel, a very sensitive subject, was taken at night to an extensive burying-ground near Vienna, where interments take place daily, and there are many thousand graves. The result did not disappoint Von Reichenbach’s expectations. Whithersoever Miss Reichel turned her eyes, she saw masses of flame. This appearance manifested itself most about recent graves. About very old ones it was not visible. She described the appearance as resembling less bright flame than fiery vapour, something between fog and flame. In several instances the light extended four feet in height above the ground. When Miss Reichel placed her hand on it, it seemed to her involved in a cloud of fire. When she stood in it, it came up to her throat. She expressed no alarm, being accustomed to the appearance. The mystery has thus been entirely solved; for it is evident that the spectral character of the luminous apparition, in the two instances which I have narrated, had been supplied by the seers themselves. So the superstition has vanished; but, as usual, it veiled a truth. LETTER IV. TRUE GHOSTS.—The apparitions themselves always sensorial illusions—The truth of their communications accounted for—Zschokke’s Seer-gift described, to show the possibility of direct mental communication—Second-sight—The true relation of the mind to the living body.