CHAPTER XIII.
OPTICAL ILLUSIONS.
The eye is easily deceived and is also very inaccurate in its judgment. In Figures 109 to 111, all of the lines are of precisely the same length, yet they appear to differ considerably. The reason for this error in the estimation of objects is not known. It is especially noticeable with such objects as a high hat; almost any one will estimate the height of a silk hat as much greater than it really is. In general, white objects also appear to be much larger than black objects. This can be seen by the two inscribed squares one black and the other white in Figure 112; both of these are of exactly the same size. Probably the fact that more light reaches the eye from a light-colored object than from a dark-colored one of equal proportions causes the impression of greater size.
That the mind has the power of mixing contradictory or conflicting impressions made upon it is proved by many facts. If a star, Figure 113, be pinned to the center of a wheel and rapidly revolved, the center of it will appear jet black; while the outer portions, made up of the points and the white background, will appear grey which will gradually fade to a lighter shade from the center black spot outward.
If a card be provided with a picture of a bird on one side and a ring on the other, Figure 114, and this be rapidly spun on one corner, after the manner of a top, the bird will appear to be in the center of the ring. This effect is due to the persistence of vision; an image formed upon the retina requires some time before it can be eliminated so that both images appear together.
This persistence of vision and the power of suggestion are made use of in a well-known act which consists of apparently throwing something, for instance a guinea pig, into the air and causing it to vanish. To perform this trick, the operator holds the pig in his hand and makes a few motions suggestive of tossing it into the air. Then, with a final more extreme motion on the downward swing of his hand, he drops the pig and swiftly moves his empty hand upward. Due to the persistence of vision, the audience actually still sees the pig and due to the suggestion of tossing the eyes look upward, and thus the persistence of vision and the power of suggestion create the illusion.
_Dissolving Views of Living Pictures._--This act requires a large glass plate of good clear quality arranged upon the stage, as shown in Figure 115. _One_ of the poses is arranged as at the black circle behind the glass and when illuminated is seen by the audience. The other is arranged at one side. The lighting of both is connected to the same dimmers in such a manner that when the light of one is increasing, that of the other is diminishing. Thus the two figures are dissolved into one another. The paths of the various rays of light to different parts of the auditorium are shown in the drawing and it is possible to procure perfect registry.
_Human Figure Floating or Performing in Air._--This act is arranged by means of a large mirror placed upon the stage as shown in Figure 116. In the _pit_ out of sight of the audience is a revolving table draped in dull black and all of the pit is draped in the same manner. A figure lying upon this table dressed in light clothes will appear erect in the mirror as indicated by the arrow. If now the table is revolved, the figure will be seen as turning over in the mirror. The figure upon the table may perform a number of evolutions suggestive of floating, flying, etc. The black cloth of the pit will reflect no light and only the figure will be visible. If a glass plate is used instead of a mirror, suitable scenery may be arranged back of it. The figure must of course be brightly illuminated. By moving the table upon which the figure rests across the pit, the figure will appear to move along.
_Head Suspended in Air._--This act is worked out in bright light. It requires a mirror with a hole in the center of it large enough to admit the head of a person, as shown in Figure 117. Above the mirror suitable decorations are provided which cause one looking at the mirror to imagine seeing the back wall of a room. The ceiling must of course be kept out of view as much as possible. A person looking on sees the head and the mirror gives him the impression of vacant space about it. The head must be surrounded by a collar or drapery of some kind so that no reflection of it will be visible.
_Magic Cabinet._--In this act a person enters the cabinet, Figure 118. The outer doors are closed for an instant and then opened. The person has disappeared! The disappearance is brought about by pulling the two mirrors, arranged inside of the cabinet, into the position indicated by the dotted lines. When the cabinet is open for inspection, these are swung to the side and are invisible. The backs are of the same design as the rest of the interior cabinet. When they are pulled together and hide the person behind them, they reflect the side walls of the cabinet and are not noticed. If the cabinet is well made and the act skilfully performed, the outer doors may even be omitted.
_Head Resting on Table._--For this act a hole large enough to allow a person’s head to project through is cut in the top of a table, Figure 119. This table has only two legs and is fitted up with mirrors, as indicated by shading. The reflection of the two legs in the mirror causes the observer to see four and imagine that the space below the table is vacant.
_Multiplication of Images._--If three large mirrors be arranged in the form of an equilateral triangle, as indicated in Figure 120, a person standing in the center will see his image reflected so many times that he will receive the impression of being in the midst of a crowd. He will see the reflections of reflections repeated until by absorption so much light is lost that they finally become invisible.
_Trick Mirror._--Rather startling effects can be produced by a thinly coated mirror _A_, Figure 121, behind which an electric light is arranged so that it may be easily turned on. The space behind the mirror being dark, no one suspects that it is anything but a common looking glass. It is, however, quite transparent when the light behind it is turned on. The person in front viewing himself may thus suddenly be brought to see anything that is behind the mirror. If another set of mirrors is arranged, as shown by shaded lines, the person in front of the mirror may suddenly be made to see a head floating in the air in front of him.
The manner in which one can see through a brick is illustrated in Figure 122. Four mirrors reflect the light around it. If the mirrors are properly arranged, the person will imagine he is looking through the brick.
The face of a person may be thrown upon a screen among clouds or other pictures in the manner shown in Figure 123. The face occupies the space marked by a half circle and two arc lamps with condensers are trained upon it at very close range. The face itself acts as a reflector projecting itself through the object lens in the center. This act is very trying on the person whose face is reflected.
At a certain distance from a concave mirror, an object will be seen inverted. If the mirror be brought closer, the image will gradually go out of focus and, by bringing the mirror still closer, it will gradually come into focus again but this time erect. Properly arranged such a mirror can be made to give weird effects, for instance a skeleton, rushing at the observer; and the image will appear to step out of the mirror.