We may also mention, while speaking on this subject, the adventure of the Emperor Alexander of Russia, _à propos_ of a singular optical experiment at which he was present, which had for its end the changing of a man into a wild animal, or _vice versâ_. Certain cynics will possibly say that this is by no means difficult, and that it is an event that happens every day; but the clever trick at which Alexander was so astonished was not moral but purely physical. After having gained much money and fame in France, Robertson directed his steps towards Hamburg, where the Emperor was at that time stopping. He performed before the Czar an experiment that puzzled his Majesty beyond endurance. He showed him a man upon whose shoulders he saw successively the head of a calf, a lion, a tiger, a bear, and a whole menagerie of other animals. At last, the Czar could stand it no longer, and he suddenly rose, put his shoulder against the partition, and brought the whole to the ground with a loud crash, just at the moment that the confederate was assuming the form of a goat. If our readers would like to join the Czar in his discovery of the manner in which the trick was performed, they can easily do so. [Illustration: FIG. 56.—Nostradamus and Marie de Médicis. ] The room in which this trick is to be performed should have a smaller one adjoining it, about eight feet square. The magician in the first place shows the small apartment to the spectator, who perceives that it contains nothing but an empty chair placed against the wall. The partition between the two rooms is provided with a small hole, covered with glass, exactly opposite the chair, and at about the ordinary height of the eyes. On the inner side there are two grooves, in which slide a block of wood containing a prism, as shown in fig. 57, which may be quickly and easily replaced by a piece of plane glass. On looking through this opening, the spectator sees a man sitting in a chair, but suddenly, without any apparent cause, the man changes into a goat, a sheep or some other animal. The sudden replacing of the prism, which takes place without the spectator perceiving it, causes him to see, not the floor with the man and chair upon it, but the ceiling, which is carpeted exactly in the same way, and is provided with a precisely similar chair, upon which is placed a goat or any other animal. [Illustration: FIG. 57.—The Arrangement of the Reversing Prism. ]