OPTICAL ILLUSIONS.
A mirror is to be made as follows: first make a hoop or fillet of wood about the width of a half-crown piece, and let the thickness be about a quarter of an inch. In the middle of this hoop fasten a bottom of wood or brass, and place several hooks of the size of small peas. Then open the one side of this bottom, set in a piece of crystal glass, and fasten it in the hoop close to the bottom;
then take a quantity of quicksilver, and put so much into the hoop as will cover the bottom; then let into the hoop another piece of crystal glass fitted thereto, and cement the sides that the quicksilver may not run out, and it is done,—the use whereof I shall not insist upon,since he that is versed in optical illusions will better conceive it, than any words of mine can either direct or assist him.
This lantern is called magical on account of the formidable apparitions that, by virtue of light, show upon the white wall of a dark room. The body of it is generally made of tin, and of the shape of a lamp. Towards the back is a concave looking-glass of metal, which may either be spherical or parabolical, and which, by a grove made in the bottom of the lantern, may be either advanced nearer or put farther back from the lamp, in which is oil or spirit of wine; and the match ought to be a little thick, that when it is lighted it may cast a good light that may easily reflect from the glass to the fore part of the lantern, where there is an aperture with a prospective in it, composed of two glasses, that make the rays converge and magnify the object.
When you mean to make use of this admirable machine, light the lamp, the light of which will be much augmented by the looking-glass at a reasonable distance. Between the fore part of the lantern and the prospective glass you have a trough, made on purpose, in which you are to run along a flat frame, with transparent colours on glass. Then all these little figures, passing successively before the prospective glass, through which passes the light of the lamp, will be painted and represented with the same colours upon the wall of a dark room, and in a gigantic and monstrous manner.
By this lantern you may show birds, beasts, and all sorts of fish, &c. &c. &c.
The Phantascope is a magic-lantern improved; that is, it will magnify or diminish the figures it is to reflect, without impairing their neatness.
This instrument is composed of a tin box, surmounted by a kind of chimney, pierced on one of its faces with a hole of a diameter corresponding with that of the cylinders, sliding into one another like the tubes of an opera-glass:this tube is adjusted by means of a slide established above the aperture made in the phantascope. The part of the adjoining cylinder is so adjusted to receive alternately wooden tablets, in which are opened cavities to contain the figures painted upon glass. The flat side of the half ball is placed in that tube, nearly close enough to touch the painted figures: this half ball is called the object glass.
In the second cylinder is found a magnifying-glass, convex on both sides, the focus whereof is double to that of the half ball, and one third of its diameter: this glass is called “the eye-glass.” It is by means of drawing those glasses close that the objects are magnified; and by removing them at a distance, that the figures are made to look smaller. Inside of the box is placed one of those lamps called “argand,” of a wide diameter, with a concave mirror, to throw as much light as possible upon the transparent figures.
To exhibit the phantasmagoria, select an apartment opening into a second room, and the door of which is at your disposal: take a wooden frame, exactly of the same dimensions as that door, upon which fasten, with brass nails, a sheet of fine linen, in preference to calico. A moment before you place the frame, wet it equally all over, to cause the thread of the cloth to swell, and to fill all interstices.
When you have proved the above-mentioned instrument, prior to your using it, make sure that the lamp does not smoke, but that it gives a vivid light; that the concave mirror is very bright; that the magnifying-glass is dry and clean. (You dry these with a piece of soft leather and some whitening, so fine as not to scratch. The painted glasses are cleaned with a piece of very fine cloth, made quite hot, or of cambric, taking care not to rub off the painting.) You then place the wooden ruler with the reversed figure in the slide, opened purposely to receive it, in the first cylinder; then, the figures being reflected on a whitewashed wall, you regulate the distance of the magnifying-glasses, in order to have the paintings seen in all their perfection at a regular fixed distance. Your instrument is then ready for the exhibition.
To this effect, you present yourself facing the framedlinen, with the phantascope suspended before you, and held by two braces across your shoulders. In order that the figures may decrease in the eyes of the spectators of the other side, you must advance slowly near the cloth, drawing the two magnifying-glasses at a distance from each other, but very gently, that the figures should remain steady.
If, on the contrary, you should wish them to appear magnified, gradually draw back, with the same precaution, and bring the glasses nearer each other; when they are quite close, the figures seem to be coming towards the spectators, and to proceed forward.
When you change the paintings, you must hide the eye-glass behind an opaque and black body. The knowledge of the management of the glasses, and the accuracy of the proceeding backward and forward, upon which depends entirely the good effect of the exhibition, may be acquired by a few days’ practice previous to the exhibition being made in company.
Notwithstanding the box for the phantascope, if made of wood, will be heavier than a tin one, yet it is to be preferred, on account of its communicating much less heat than the tin.
Such persons as wish to paint their figures should use a varnish that does not spread on the glass; thick copal varnish will answer the purpose.
Make a box of wood, of a cubical figure, of about fifteen inches every way. Let it be fixed on a pedestal, at the usual height of a man’s head. In each side of this box, let there be an opening of an oval form, ten inches high, and seven wide.
In this box place two mirrors, with their backs against each other; let them cross the box in a diagonal line, and in a vertical position. Decorate the openings in the sides of this box with four oval frames, and transparent glasses, and cover each of them with a curtain, so contrived that they may all draw up together.
Place four persons in front of the four sides, and atequal distances from the box, and then draw up the curtains, that they may see themselves in the mirrors; when each of them, instead of his own figure, will see that of the person who is next him, and who, at the same time, will seem to him to be placed on the opposite side. Their confusion will be the greater, as it will be very difficult for them to discover the mirror contained in the box. The reason of this phenomenon is evident; for, though the rays of light may be turned aside by a mirror, yet they always appear to proceed in right lines.
In a partition make two apertures of a foot high, and ten inches wide, and about a foot distant from each other. Let them be at the common height of a man’s head; and in each of them place a transparent glass, surrounded with a frame, like a common mirror.
Behind this partition place two mirrors, inclined to it in an angle of forty-five degrees (that is, be halfway between a line drawn perpendicular to the ground and its surface); let them be both eighteen inches square; let all the space between them be enclosed by boards, or pasteboard, painted black, and well closed, that no light may enter. Let there be also two curtains to cover them, which may be drawn aside at pleasure.
When a person looks into one of these supposed mirrors, instead of seeing his own face, he will perceive the object that is in front of the other; so that, if two persons present themselves at the same time before these mirrors, instead of each one seeing himself, they will reciprocally see each other.
Note.—There should be a sconce, with a candle placed on each side of the two glasses in the wainscot, to enlighten the faces of the persons who look in them; otherwise this experiment will have no remarkable effect.
This curious trick may be considerably improved, by placing the two glasses in the partition in adjoining rooms; and, a number of persons being previously placed in one room, when a stranger enters the other, you may tell him his face is dirty, and desire him to look in theglass, which he will naturally do; and on seeing a strange face he will draw back; but, returning to it, and seeing another, and another, like the phantom kings in Macbeth, what his surprise will be is more easy to conceive than express. After this, a real mirror may be privately let down on the back of the glass, and, if he can be prevailed to look in once more, he will then, to his further astonishment, see his own face, and may be told, perhaps persuaded, that all he thought he saw before was mere imagination.
How many tricks, less artful than this, have passed in former times for sorcery, and pass at this time, in some countries, for apparitions!
Note.—When a man looks in a mirror that is placed perpendicular to another, his face will appear entirely deformed. If the mirror be a little inclined, so as to make an angle of eighty degrees (that is, one ninth part from the perpendicular), he will then see all the parts of his face, except the nose and forehead. If it be inclined to sixty degrees (that is, one third part), he will appear with three noses and six eyes: in short, the apparent deformity will vary at each degree of inclination; and when the glass comes to forty-five degrees (that is, half way down), the face will vanish. If, instead of placing the two mirrors in this situation, they are so disposed that their junction may be vertical, their different inclinations will produce other effects, as the situation of the object relative to these mirrors is quite different.
Behind a partition place, in a position somewhat oblique, a concave mirror, which must be at least ten inches in diameter, and its distance from the partition equal to three fourths of the distance of its centre.
In the partition, make an opening of seven or eight inches, either square or circular: it must face the mirror, and be of the same height with it. Behind this partition place a strong light, so disposed, that it may not be seen at the opening, and may illumine an object placed near it, without throwing any light on the mirror.
Beneath the aperture in the partition, place an objectthat you intend shall appear on the outside of the partition, in an inverted position, and which we will suppose to be a flower. Before the partition, and beneath the aperture, place a little flower-pot, the top of which should be even with the bottom of the aperture, that the eye may see the flower in the same position as if its stalks came out of the pot.
Take care to paint the space between the back part of the partition and the mirror black, to prevent any reflections of light from being thrown on the mirror; in a word, so dispose the whole that it may be as little enlightened as possible.
When a person is placed at the point of vision, he will perceive the flower that is behind the partition, at the top of the pot; but, on putting out his hand to pluck it, he will find that he attempts to grasp a shadow.
Observations.
The phenomena that may be produced by means of concave mirrors are highly curious and astonishing. By their aid spectres of various kinds may be exhibited. Suppose, for example, you were to tell any one, that at such an hour, and in such a place, he should see the apparition of an absent or deceased friend (of whose portrait you are in possession); in order to produce this phantom, instead of the hole in the partition, there must be a door which opens into an apartment to which there is a considerable descent. Under that door you are to place the portrait, which must be inverted and strongly illuminated, that it may be properly reflected by the mirror, which must be large and well polished. Then, having introduced the spectator at another door, and placed him in the proper point of view, you suddenly throw open the first door, when, to his great astonishment, he will immediately see the apparition of his friend.
It will be objected, perhaps, that this is not a perfect apparition, because it is only visible at one point of view, and by one person. But it should be remembered, that it was an established maxim in the last century, that a spectre might be visible to one person, and not toothers. So Shakespeare makes both Hamlet and Macbeth see apparitions that were not visible to others, present at the same time. It is not unlikely, moreover, that this maxim took its rise from certain apparitions of the kind,that were raised by the monks, to serve some purposes they called religious; as they alone were in possession of what little learning there then was in the world.
Draw any thing you may fancy on a thin white pasteboard; then prick it; afterwards put the same on an horizontal surface, which we will suppose to be another pasteboard; put a lighted candle behind that drawing, and draw on the horizontal surface the lines given by the light; this will give a deformed design. This being done, take away the drawing that was pricked, and the candle; then place your eye where the light was, and you will see your drawing assume a regular form.