Among the more innocent illusions of art, I may remind you of concave and cylindrical mirrors and lenses, the magic lanthorn, “les ombres chinoises,”and the phantasmagoria of Cagliastro, by which daggers appear to strike the breast of the spectator, and images of objects in other rooms are thrown on the walls of that in which we are sitting. A mirror, thus accidentally placed, has afforded the evidence of murder within our own time. The duration of impressions on the eye, is another source of illusion. An image remains on the retina, I believe, about the eighth of a second; as it departs, if another object supplies its place in quick succession, the two images form, as it were, a union, and become blended. A knowledge of this law, in the ages of blind superstition, would have placed an overwhelming weapon in the hands of priestcraft; in our day, it is the source of rational and innocent pleasure, by the invention of optical toys. The whisking of an ignited stick produces a fiery circle—why? Because from _excessive rapidity_ the rays from one point remain impressed on the retina, until the revolution completes the circle. The _Thaumatrope_, or wonder-turner, and the Phantasmascope, are ingenious illustrations of this law of impression; so also is the _whirling machine_, which so beautifully evinces the fact of white being compounded of all the prismatic colours, blended in certain proportions. The prismatic Iris is painted on a revolving circle; by excessive rapidity of revolution, the colours are actually blended (as if mixed in a vessel) on the retina, and the surface of the machine is white to the eye. To these may be added the combustion of phosphorus and other substances, in oxygen: red, green, and blue lights, which change the angel face of beauty into the visage of a demon; and the inhalation of noxious fumes and gases, creating altogether a new train of phantoms in the world of experimental magic, and developing the formerly occult mysteries of the art of incantation. Chance may also involve a seeming mystery of very awful import. Some years ago the town of Reading was thus bewildered. On the loaves were seen the most mysterious signs. On one, a skeleton’s head and cross-bones; on another, the word “resurgam;” on another, a date of death was marked in deep impressions. The loaves of course were, by some mysterious influence, the vehicles of solemn warning from the Deity. The baker was _churchwarden_ of St. Giles’s; his oven needed flooring, and, winking at the sacrilege, he stole the flat inscribed tombstones from the church-yard, and therewith floored his oven. From the inscriptions of these stones the loaves took their mystic impressions.