Of all the Errors with which the Brains of Mankind have been troubled, there is none of such ancient Standing as this. We have Ghosts and Hobgoblins even in Homer; not that there is Reason to suspect that Homer ever believed in them himself; he seemed too well versed in the real Phoenomena of Nature, to entertain any such chimerical Dreams as Truth; for Dreams they are, and no better: the true _Somnia Vatum_ of the Ancients, handed down to Posterity, even to these enlightened Times. How many horrible Nights have been passed in cold Sweats, by otherwise very sensible People, owing to nothing else but the Apprehensions which they have had of these no-existing Gentry! How was even the Metropolis itself terrified the other Day, by the Scratching Ghost at Cock Lane! I think enough has been said, even in this little Book, to prove that no Noise can be made, unless by the Vibration of some elastic Body. If a Noise is made by a Voice, it must be from an Animal, which has Lungs and Breath to do it; if a Scratching is made, it must be done by something which has Hands, and Sinews to move those Hands; and it must have Nails, or some other bodily Substance, to scratch with, before it can cause a Sound to proceed from an elastic Body. So much for Scratching, and dismal Yellings, and Groanings of all Sorts, which have been fabled of Ghosts. It will require a little Dissertation upon Optics, in order to explain the Cause of Light and Colours, before we can confute the Possibility of seeing an Apparition. Light is found to be a real Substance; it is swift beyond Comprehension; if I mistake not, it is calculated by Sir Isaac Newton, to be only eight Minutes in passing from the Sun to the Earth; it is very subtle, passing through the hardest transparent Bodies; it is capable of Refraction and Reflection, that is, either of passing through a transparent Body, as a Window, or of being reflected from an even Surface, as a Looking-Glass, or a Piece of polished Steel; so that if we see any Object at my Time, the Cause of our seeing it, if there is no Window between, is by Reflection, or by the Rays of Light being reflected from the Object to the Eye of the Beholder, which is formed for the Reception of the Rays which come from the Object, in the same Manner as a Camera obscura.