# The wonders of optics/Other optical illusions

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OTHER OPTICAL ILLUSIONS.
By varying the disposition of mirrors, prisms, lenses, and light, an infinite number of the most surprising effects may be shown, with a comparatively small amount of trouble and expense. We shall, therefore, devote this chapter to the explanation of a large number of allusions, which have been devised by Robertson and other adepts in the art of honest deception.
One of Robertson's most famous delusions was the "Dance of Demons," an effect he discovered quite accidentally. One evening, while experimenting with the phantasmagoria, he suddenly found himself in the dark, when two persons, each bearing a light, crossed the room on the other side of the screen. A little window which happened to be between the lights and the screen, immediately threw its double image on the cloth, and the method of multiplying shadows was discovered.
The figures used in this experiment are cut out of fine cardboard, and may be made a foot high or thereabouts. They are placed on a second screen in front of the principal one, and by multiplying the lights, as shown in fig. 55, you may have as many shadows as you please. The effect is much heightened if the figures are cut out so as to show as lights when thrown on the screen. A little ingenuity shown in the arrangement of the distance and movements of the lights, will produce an ​
Fig. 55.—Wizard Dance.
of the principal figure may be produced by carrying the second light to a great distance, and the lesser figure may be easily made to jump over the former, by moving the candle in a semicircle over the light that is stationary. It is only necessary to recollect that whatever movements are made by the lights, the shadows of the figures follow their example. With a little ingenuity the heads and limbs of the figures may be made moveable; and if one assistant attends entirely to the working of the figures, and the rest to the lights, an infinite number of changes may be carried out. If mounted in a frame, they may be made to throw somersaults, fall down, or jump up in the air at will.
A knowledge of optics will often serve to explain with great ease the tricks played by conjurers and impostors on princes and other great people, for their own vile ends. It is well known that Nostradamus, on being consulted by Marie de Médicis on the future destiny of France, was shown by him in a mirror events that left no doubt on her mind that she would one day share the throne of the Bourbons. These illusions were possibly effected in the following manner, and may be readily understood by reference to fig. 56.
The throne in the first chamber is reflected in a mirror concealed in the canopy overshadowing a second mirror, placed carelessly on a table in the room in which the Princess and astrologer are standing. The arrangement of the mirrors is such that, on looking into the smaller glass, the Princess sees all that is going on in the adjoining chamber. The very fact of her consulting Nostradamus on her future fate, shows that under certain circumstances, at least, this clever woman was as silly as a child. It is not, therefore, to be supposed that she would notice that the mirror she was looking into was inclined at such an angle that it could not reflect her ​beautiful face. Nothing could be more natural, either, than that this magic looking-glass should be placed om a daïs, and shaded by a canopy. Nostradamus, who was a shrewd man, could no doubt pretty well see the course that events would take, and must consequently have felt quite safe in showing the Princess the throne of France occupied by Henry of Navarre. This was not the first time that the rulers of the earth were duped by so-called magicians, who possessed the knowledge that the angle of reflection was always equal to the angle of refraction.
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.
The room in which this trick is to be performed should have a smaller one adjoining it, about eight feet square. ​
Fig. 56.—Nostradamus and Marie de Médicis.
​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, Fig. 57.—The Arrangement of the Reversing Prism. 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.
While looking at the goat, the plane glass is substituted for the prism, and the man reappears; another ​movement of the prism, and he changes into a sheep, a figure of a sheep having in the meantime replaced that of the goat. Of course it is necessary not merely to have the walls, floors, and chairs precisely alike, but they must each occupy the same relation to each other. If it is desirable only to change the head, it is simply necessary to have a lay figure with a moveable head, dressed precisely in the same manner as the living operator, in the upper portion of the chamber. At the end, by the substitution of the empty chair, the individual may be made to disappear entirely.
There may often be seen in the streets of London, a man showing a wonderful instrument, consisting of a telescope cut in two, the two portions being separated from each other by an interval of three or four inches. On looking through the instrument, the spectator of course sees the object at which it is pointed; but what is his astonishment to find, that when the showman places a brick between the two halves of the instrument he sees just as well as before. The showman generally informs him that the instrument in question has such powerful lenses, that it will not only see through a brick, but even through a policeman's head if it happened to be in the way; and the spectator, having paid his penny, goes away perfectly mystified, until, like the young lady who believed that all machinery was worked "by a screw, somehow," he comforts himself with the idea that the trick is performed "by a mirror, somehow." The following figure will, however, soon clear up the mystery.
Let F M, L G be an ordinary telescope tube, to be separated in the middle by an interval large enough to insert a brick, the hand, or some other opaque object. The whole is fixed on a stand, consisting of a square tube with a couple of elbows to it. Between G and L a mirror (A) is placed diagonally, which receives the ​
Fig. 58.—The Goat Trick.
​image of the objects to be looked at. This mirror sends the image downwards to another placed diagonally at C, a third being placed at D, and a fourth at B. The Fig. 59.—How to see through a Brick. horizontal ray, meeting the mirror at A, is consequently bent downwards to C, then travels horizontally to D, when it is reflected upwards to B, in which it is seen by the eye. Of course a simple tube without any lenses at all would serve the same purpose, but the fact of its being a telescope serves to distract the attention of the too curious observer.
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.
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 Hel-​
Fig. 60.—The Polemoscope.
​vetius, about 1637. Fig. 60 will show the principle of this instrument.
The luminous rays coming from a distant object are received upon an inclined mirror, which is elevated above the parapet of a fortification, and are reflected downwards to a second, which is placed at a corresponding angle. If necessary, lenses can be interposed, so as to give a magnified view of the distant object that is being examined. By means of such an instrument, the movements of the enemy can be followed without danger, the apparatus being generally of small size, and not attracting notice. Amongst the varieties of this instrument, is one whose use is readily seen by inspecting fig. 61, by which it seems to be perfectly possible to see with safety all that is going on outside the door of the house without being perceived. The line of the mirrors in this case is at right angles to that of the polemoscope in fig. 60. Amongst the different varieties of polemoscope which have been invented, may be mentioned a reflecting opera-glass, which was greatly used by the beaux and dandies of the last century. In the tube of this instrument was inserted an inclined mirror, which allowed the spectator to point his glass in quite a different direction to that of the object he was really looking at. In fact, it was constructed somewhat on the same principle as the Herschellian or Newtonian telescope, and enabled the possessor, while apparently enjoying the play, to observe all that was going on in the boxes or pit of the theatre. Years ago, there was a little instrument of a similar kind, sold for a penny in the streets of London, which consisted of a morsel of looking-glass set at an angle, in a pill-box, and which gave the possessor the power of seeing all that was going on behind him. Persons who wear dark preservers are often in the habit of observing all that is ​going on behind their backs by the reflection seen in the corner of their glasses.
Such are the principal optical recreations founded on the reflecting and refracting properties of mirrors and lenses. We shall end this chapter by appending to it the description of a few additional optical amusements that are quite within the reach of the amateur.
If the reader is in possession of a concave mirror, it may be made the means of performing a number of amusing experiments. In front of it is placed a plaster head, a skull or any other object, mounted on wheels and running along a grooved platform, which is naturally kept perfectly concealed from the spectators. The mirror is slightly inclined, so as to reflect the image of the object at an angle to the observer's eye. By running the cast backwards and forwards, it will have the appearance of advancing and retiring from the spectator in a very imposing manner. A dagger may be substituted for the cast, and by being made to work up and down on a pivot, will have the appearance of striking at the spectator. We have already seen that an experiment of this sort had such an effect on Louis XIV. that he drew his sword to defend himself from his imaginary aggressor. There is another way of performing this trick, by suddenly illuminating the skull or dagger by means of a dark-coloured box containing a light, which may be made to throw its reflections on the object, by sliding it along a couple of wires. In the case of the dagger, however, the hinged arrangement will be found more effective.
One of Robertson's tricks was called the "Magic Box," and he astonished a numerous party of visitors who were staying at a country house to which he had been invited. One of the gentlemen who was always boasting of his freedom from superstitious feelings of any kind, had had several arguments with Robertson on ​
Fig. 61.—Protection against ill-natured people.
he would at any rate surprise his strong-minded friend by an easy trick or two. He consequently chose as his confederate a lady to whom the gentleman had been paying great attention during the time of his visit. Robertson one evening mysteriously delivered a small box to him, which he was to place upon his toilet table, and unlock exactly at midnight. The gentleman did so, and what was his astonishment to see the face of the lady with whose charms he had been so deeply impressed suddenly spring out of the box. His look of terror and surprise was evidently too much for Robertson's confederate, who burst into a merry peal of laughter, leaving her admirer in a very disconcerted state.
After all we have said on the subject of mirrors, it is not difficult to guess how this trick was performed. The box in question was painted black on the inside, and contained a concave mirror placed at an angle of 45°. The reflection of the lady, who was of course in the next room, was carried by means of several plane mirrors placed in boxes communicating with each other through the partition of the room, the head of the lady only being strongly illuminated, the rest of her figure not appearing by being kept quite dark.
The figures reflected from smoke are extremely surprising. To perform such experiments a phantasmagoria is necessary. The focus is so adjusted that the distant image falls just above a brasier containing lighted charcoal. Everything being ready, a few grains of olibanum or other gum are thrown on the coals, and the smoke that rises immediately affords a screen for the reflection of the images proceeding from the phantasmagoria. If the amateur is not the possessor of a magic lantern, a properly arranged concave mirror will answer almost the same purpose.