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.
[Illustration:
FIG. 55.—Wizard Dance.
]
The figures used in this experiment are cut out of fine cardboard, and
may be made a foot high or there-abouts. 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 endless
amount of amusing effects. Thus, a small image 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 on 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.
[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.
]
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.
[Illustration:
FIG. 58.—The Goat Trick.
]
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 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
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.
[Illustration:
FIG. 59.—How to see through a Brick.
]
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.
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.
[Illustration:
FIG. 61.—Protection against ill-natured people.
]
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 the subject of apparitions, and the latter
thought that 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.
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