Another illusion of the same kind is often practised at fancy fairs and bazaars, when a spectator looking into what he supposes to be an ordinary looking-glass, sees his companions instead of himself. The way in which this is effected is very simple. A looking-glass is placed diagonally across a square box, the apertures in the sides being so arranged that the spectator does not perceive that he is looking into a glass that is placed at an angle. Of course the exhibitor endeavours to show the illusion to two persons at once; and if they are strangers to each other, and of the opposite sex, a great deal of fun is made out of the trick. A showman at Greenwich made an immense harvest by showing two such mirrors, one to all the young girls who wished to see their future husbands, and the other to all the young men who wished to see their future wives. Of course he had a tolerably good-looking male and female confederate to help him. With a couple of mirrors placed back to back in a square case, with an opening on each side, the illusion is still more perfect, as on looking through any of the holes the box seems to be quite empty. The “Speaking Head” trick is performed on this principle. When the curtain is drawn up, the audience perceive an apparently living head placed on a small three-legged table, the curtain at the back of the stage being quite visible through the legs. By and by the bodiless head, which is generally painted in a very fantastic manner, begins to speak, answers questions, and ends by singing a song. The trick is performed in the following way: The spaces between the legs are filled with a looking-glass; consequently, the spectators see the reflection of the curtains at the _sides_ of the stage, which are made exactly like those at the back, thus giving the table the appearance of standing on three slim legs, with nothing between. Behind the looking-glass there is of course plenty of space for the body of the man belonging to the magical head. The exhibitor naturally takes especial care never to pass in front of the table, otherwise the lower part of his body would be reflected in mirrors. [Illustration: FIG. 60.—The Polemoscope. ] The polemoscope (from two Greek words signifying “war” and “to see”) is another instance of double reflection. It was said to have been invented by Helvetius, about 1637. Fig. 60 will show the principle of this instrument.