In order to render the images which are formed by the glass and water prisms as perfect as possible, it would be easy to make them achromatic, and the figures might be multiplied to any extent by using several prisms, having their refracting edges parallel, for the purpose of giving a similarity of position to all the images. [Illustration: _Fig. 9._] Among the instruments of natural magic which were in use at the revival of science, there was one invented by Kircher for exhibiting the mysterious hand-writing on the wall of an apartment, from which the magician and his apparatus were excluded. The annexed figure represents this apparatus as given by Schottus. The apartment in which the spectators are placed is between LL and GH, and there is an open window in the side next LL, GH being the inside of the wall opposite to the window. Upon the face of the plane speculum EF are written the words to be introduced, and when a lens LL is placed at such a distance from the speculum, and of such a focal length, that the letters and the place of their representation are in its conjugate foci, a distinct image of the writing will be exhibited on the wall at GH. The letters on the speculum are of course inverted, as seen at EF, and when they are illuminated by the sun’s rays S, as shown in the figure, a distinct image, as Schottus assures us, may be formed at the distance of 500 feet. In this experiment, the speculum is by no means necessary. If the letters are cut out of an opaque card, and illuminated by the light of the sky in the day, or by a lamp during night, their delineation on the wall would be equally distinct. In the daytime it would be necessary to place the letters at one end of a tube or oblong box, and the lens at the other end. As this deception is performed when the spectators are unprepared for any such exhibition, the warning written in luminous letters on the wall, or any word associated with the fate of the individual observer, could not fail to produce a singular effect upon his mind. The words might be magnified, diminished, multiplied, coloured, and obliterated, in a cloud of light, from which they might again reappear by the methods already described, as applicable to the magic lantern. The art of forming aërial representations was a great desideratum among the opticians of the 17th century. Vitellio and others had made many unsuccessful attempts to produce such images, and the speculations of Lord Bacon on the subject are too curious to be withheld from the reader.