Part 17
Between the phantasmagoria and the magic lantern there is this difference; in common magic lanterns the figures are painted on transparent glass, consequently the image on the screen is a circle of light having figures upon it; but in the phantasmagoria all the glass is made opaque, except the figures, which, being painted in transparent colors, the light shines through them, and no light can come upon the screen except that which passes through the figure, as is here represented.
There is no sheet to receive the picture, but the representation is thrown on a thin screen of silk or muslin placed between _the spectators and the lantern_. The images are made to appear approaching and receding by removing the lantern further from the screen, or bringing it nearer to it. This is a great advantage over the arrangements of the magic lantern, and by it the most astonishing effects are often produced.
DISSOLVING VIEWS.
The dissolving views, by which one landscape or scene appears to pass into the other while the scene is changing, are produced by using two magic lanterns placed side by side, and that can be a little inclined towards each other when necessary, so as to mix the rays of light proceeding from the lenses of each together, which produces that confusion of images, in which one view melts as it were into the other, which gradually becomes clear and distinct.
HOW TO RAISE A GHOST.
The magic lantern or phantasmagoria, may be used in a number of marvelous ways, but in none more striking than in raising an apparent specter. Let an open box, A B, about three feet long, a foot and half broad, and two feet high, be prepared. At one end of this place a small swing dressing glass, and at the other let a magic lantern be fixed with its lenses in a direction towards the glass. A glass should now be made to slide up and down in the groove C D, to which a cord and pulley should be attached, the end of the cord coming to the part of the box marked A. On this glass the most hideous specter that can be imagined may be painted, but in a squat or contracted position, and when all is done, the lid of the box must be prepared by raising a kind of gable at the end of the box B, and in its lower part at E, an oval hole should be cut sufficiently large to suffer the rays reflected from the glass to pass through them. On the top of the box F place a chafing dish, upon which put some burning charcoal. Now light the lamp G in the lantern, sprinkle some powdered camphor or white incense on the charcoal, adjust the slide on which the specter is painted, and the image will be thrown upon the smoke. In performing this feat the room must be darkened, and the box should be placed on a high table, that the hole through which the light comes may not be noticed.
THE THAUMATROPE.
This word is derived from two Greek words, one of which signifies _wonder_, and the other _to turn_. It is a very pretty philosophical toy, and is founded upon the principle in optics, that an impression made upon the retina of the eye lasts for a short interval after the object which produced it has been withdrawn. The impression which the mind receives lasts for about the eighth part of a second, as may be easily shown by whirling round a lighted stick, which if made to complete the circle within that period, will exhibit not a fiery point, but a fiery circle in the air.
THE BIRD IN THE CAGE.
Cut a piece of cardboard of the size of a penny piece, and paint on one side a bird, and on the other a cage; fasten two pieces of thread, one on each side, at opposite points of the card, so that the card can be made to revolve by twirling the threads with the finger and thumb; while the toy is in its revolution, the bird will be seen within the cage. A bat may in the same manner be painted on one side of the card, and a cricketer upon the other, which will exhibit the same phenomenon, arising from the same principle.
CONSTRUCTION OF THE PHANTASMASCOPE.
The above-named figure is a Thaumatrope, as much as the one we are about to describe, although the term Phantasmascope is generally applied to the latter instrument; which consists of a disc of darkened tin-plate, with a slit or narrow opening in it, about two inches in length. It is fixed upon a stand, and the slit placed upwards, so that it may easily be looked through. Another disc of pasteboard, about a foot in diameter, is now prepared and fixed on a similar stand, but with this difference, that it is made to revolve round an axis in the center. On this pasteboard disc, paint in colors a number of frogs in relative and progressive positions of leaping; make between each figure a slit of about a quarter of an inch deep; and when this second disc is made to revolve at a foot distance behind the first, and the eye is placed near the slit, the whole of the figures, instead of appearing to revolve with the disc, will all appear in the attitudes of leaping up and down, increasing in agility as the velocity of the motion is increased. It is necessary, when trying the effect of this instrument, to stand before a looking-glass, and to present the painted face of the machine towards the glass.
A very great number of figures may be prepared to produce similar effects--horses with riders in various attitudes of leaping, toads crawling, snakes twisting and writhing, faces laughing and crying, men dancing, jugglers throwing up balls, &c.; all of which, by the peculiar arrangement above detailed, will seem to be in motion. A little ingenuity displayed in the construction and painting of the figures upon the pasteboard disc will afford a great fund of amusement.
CURIOUS OPTICAL ILLUSION.
One of the most curious facts relating to the science of vision is the absolute insensibility of a certain portion of the retina to the impression of light, so that the image of any object falling on that point would be invisible. When we look with the right eye, this point will be about fifteen degrees to the right of the object observed, or to the right of the axis of the eye, or the point of most distinct vision. When looking with the left eye, the point will be as far to the left. The point in question is the basis of the optic nerve, and its insensibility to light was first observed by the French philosopher, Mariotte. This remarkable phenomenon may be experimentally proved in the following manner:
Place on a sheet of writing-paper, at the distance of about three inches apart, two colored wafers; then, on looking at the left-hand wafer with the right eye, at the distance of about a foot, keeping the eye straight above the wafer, and both eyes parallel with the line which forms the wafers, the left eye being closed, the right-hand wafer will become invisible; and a similar effect will take place if we close the right eye, and look with the left.
ANOTHER.
Cut a circular piece of white paper, about two inches in diameter, which affix to a dark wall. At the distance of two feet on each side, but a little lower, make two marks; then place yourself directly opposite the paper, and hold the end of your finger before your face, so that when the right eye is open it shall conceal the mark on your left, and when the left eye is open the mark on your right. If you then look with both eyes at the end of your finger, the paper disc will be invisible.
ANOTHER.
Fix a similar disc of paper, two inches in diameter, at the height of your eye on a dark wall; a little lower than this, at the distance of two feet on the right hand, fix another of about three inches in diameter; now place yourself opposite the first sheet of paper, and, shutting the left eye, keep the right eye still fixed on the first object, and when at the distance of about ten feet, the second piece of paper will be invisible.
Or, fix three pieces of paper against the wall of a room, at equal distances, at the height of the eye. Place yourself directly before them, at a few yards distance, close your right eye, and look at them with your left, when you will see only two of them, suppose the first and second; alter your position a second time, and you will see the second and third, but never the whole three pieces together.
On a sheet of black paper, or other dark ground, place two white wafers, having their centers three inches distant. Vertically above the paper, and to the left, look with the right eye, at twelve inches from it, and so, that when looking down on it, the line joining the two eyes shall be parallel to that joining the center of the wafers. In this situation close the left eye, and look full with the right perpendicularly at the wafer below it, when this wafer only will be seen, the other being completely invisible. But if it be removed ever so little from its place, either to the right or left, above or below, it will become immediately visible, and start, as it were, into existence. "It will cease to be thought singular," says Sir John Herschel, "that this fact of the absolute invisibility of objects in a certain point of the field of view of each eye should be one of which not one person in ten thousand is apprized, when we learn that it is not extremely uncommon to find persons who have for some time been totally blind with one eye without being aware of the fact."
THE PICTURE IN THE AIR.
One of the numerous optical illusions which have, from time to time, been evolved by scientific minds, is that of making an image or picture appear in the air. This is produced by means of a mirror, and an object in relief, upon which a strong light is thrown--the mirror being set at such an angle as to throw up the reflection of the image to a certain point, in the view of the spectator. This illusion is produced as follows: Let a screen be constructed in which is an arched aperture, the center of which may be five feet from the floor; behind the screen is placed a large mirror of an elliptical form. An object is now placed behind the screen, upon which the light of a strong lamp is thrown from a point above the mirror, and is received by the mirror and reflected to the center of the arched cavity in the screen, where it will appear to the spectator. Care should be taken to place the image in an inverted position, and the light should be so placed that none of it may reach the opening: the light must also be very powerful.
BREATHING LIGHT AND DARKNESS.
The following experiment, if performed with care, is exceedingly striking. Let S be a candle, whose light falls at an angle of 56° 45´ upon two plate glasses, A B, placed close to each other; and let the reflected rays, A C, B D, fall at the same angle upon two similar plates, C D, but so placed that the plane of reflection from the latter is at right angles to the plane of reflection from the former. An eye placed at E, and looking at the same time on the two plates, C and D, will see very faint images of the candle S; which by a slight adjustment of the plates, may be made to disappear almost wholly, allowing the plate C to remain where it is. Change the position of D till its inclination to the ray B D is diminished about 3°, or made nearly 53° 11´. The distance may be easily found by a little practice. When this is done, the image that had disappeared on looking into D, will be restored, so that the spectator at E, upon looking into the two mirrors, C D, will see no light in C, because the candle has nearly disappeared, while the candle is distinctly seen in D. If, while the spectator is looking into these two mirrors, either he or another person breathes upon them gently and quickly, the breath will revive the extinguished image in C, and will extinguish the visible image in D.
EXPLANATION.--The light A, C, B, D is polarized by reflection from the plates A B, because it is incident at the polarizing angle 56° 45´ for glass. When we breathe upon the plates C D, we form upon their surface a thin film of water, whose polarizing angle is 53° 11´, so that if the polarized rays A C, B D, fall upon the plates C D, at an angle of 53° 11´, the candle from which they proceed would not be visible, or they would not suffer reflection from the plates C D. At all the other angles the light would be reflected, and the candle visible. Now the plate D is placed at an angle of 53° 11´, and C at an angle of 56° 45´, so that when a film of water is breathed upon them, the light will be reflected from the latter, and none from the former; that is, the act of breathing upon the glass plates will restore the invisible, and extinguish the visible image.
TO SHOW WHAT RAYS OF LIGHT DO NOT OBSTRUCT EACH OTHER.
Make a small hole in a sheet of pasteboard A, and placing it upright before three candles B, placed closely together, it will be found that the images of all the candle flames will be formed separately on a piece of paper C, laid on the table to receive them. This proves that the rays of light do not obstruct each other in their progress, although all cross in passing through the hole.
HOW TO SEE THROUGH A PHILADELPHIA BRICK.
Construct a hollow box or case, like the figure in the margin. One side is purposely removed in the engraving, to enable you to see the arrangement of the interior, A, B, C, and D, are four small pieces of looking-glass, all placed at an angle of 45°, with respect to those sides of the box on which they are fixed; at E and G, two flat pieces of glass are inserted, as in the eye-glass of a telescope. Supposing you look through the opening E, in the direction of an object placed at O, you would see it in the same manner as if there was an uninterrupted view between E and G, which is evidently not the case. The cause of this is readily explained. The image of the object at O is received on the looking-glass A, by which it is reflected to B, as it is again from B to C, and afterwards to D; and this last image in D is seen by the eye of the spectator placed at E, in the same direction as if in reality he was looking at the real object itself, in the direction of the dotted line from O to E. From this it is evident that the placing an opaque body at F, cannot prevent the object at O being seen. Of course all this arrangement of the instrument is concealed, and you place it in the hands of a companion, that he may look through E or G, it matters not which, at any object placed beyond. You may then safely lay a wager that your instrument is of so magical a nature that it will enable you to see through a brick wall; but as a single brick will be more convenient, and equally wonderful, you are willing to satisfy his doubts at once. Of course, the hand or the hat, or any other opaque object, will answer the same purpose.
THE STEREOSCOPE.
This is one of the newest and most interesting optical surprises invented, but, like many other instruments, it is indulged with a very hard name, which means, "Solids I see." For mere amusement, the instrument supplied with the necessary pictures may suffice, and they are now sold like the kaleidoscope at all optical instrument makers, toyshops, &c. The effect consists in obtaining the perfect solidity of a geometric object from two ordinary drawings, pictures of columns, statuary, figures, flowers, &c., &c., having a rounded appearance, "breadth," and keeping, which induce the spectator to believe he is gazing at the natural figure. Our limits preclude a lengthened description of the philosophy of this instrument, the invention of Professor Wheatstone. We may, however, first recommend our readers to study the structure of the eye, in Brewster's Treatise on Optics, which may be thoroughly impressed on the mind by dissecting carefully the eye of a sheep or bullock. Now, if we cut open a portion of the eye of a recently killed animal, and look in upon the retina, which is a delicate network of nerves, and is considered the "mind of the eye," we shall behold all images inverted. How then, do we see them upright? Again, as we have two eyes at a distance from each other, the images formed on the two retinæ cannot be precisely alike: how is it that confusion is not the result, instead of perfect images, in which we can appreciate the geometric niceties of length, breadth, and thickness? Now the stereoscope assists us in understanding these difficult questions; and, quoting Professor Wheatstone, we find he states "that the theory which has obtained the greatest currency is that which assumes that an object is seen single because its pictures fall on corresponding points of the two retinæ; that is, on points which are similarly situated with respect to the two centers, both in distance and position. This theory supposes that the pictures projected on the retinæ, are exactly similar to each other corresponding points of the two pictures falling on corresponding points of the two retinæ."
Now, the fact is, that an object presents an entirely different appearance to each eye. Sir D. Brewster remarks, "That were a painter called upon to take drawings of a statue, as seen by each eye, he would fix at the height of his eyes a metallic plate, with two small holes in it, and he would then draw the statue as seen through the holes by each eye." With the utmost care, however, he could not reproduce the statue by their union. In order to do this, a camera with _two lenses_ of the same aperture and focal length, placed at the same distance as the _two eyes_, must be constructed and used.
The stereoscope is, consequently, an imitation of the powers of the eyes, giving solidity and a perfect relievo appearance to any two pictures which might be drawn separately from the two lenses in the camera obscura mentioned. That is to say, if it were possible to be behind the retina of each eye, and draw the two pictures of any object seen by our eyes, those pictures put into the stereoscope, would reproduce the solidity from which they were drawn.
Two instruments are sold, and may be obtained with the photographic pictures, almost at any optician's, viz.: the reflecting and the refracting stereoscope, of which we give drawings.
_a_ _a_, the two pictures, B B, the two mirrors, so adjusted that their backs form an angle of ninety degrees with each other, i. e., the quarter of a circle.
OCULAR SPECTRA.
One of the most curious affections of the eye is that in virtue of which it sees what are called _ocular spectra_, or accidental colors. If we place a red wafer on a sheet of white paper, and, closing one eye, keep the other directed for some time to the center of the wafer, then, if we turn the same eye to another part of the paper, we shall see a green wafer, the color of which will continue to grow fainter and fainter, as we continue to look at it.
By using differently-colored wafers, we obtain the following results:
WAFER. SPECIMEN. Black, White. White, Black. Red, Bluish Green. Orange, Blue. Yellow, Indigo. Green, Violet, with a little Red. Blue, Orange Red. Indigo, Orange Yellow. Violet, Bluish Green.
BRILLIANT WATER MIRROR.
Nearly fill a glass tumbler with water, and hold it, with your back to the window, above the level of the eye, as in the engraving. Then look obliquely, as in the direction E, _a_, _c_, and you will see the whole surface shining like burnished silver, with a strong metallic reflection; and any object, as a spoon, A, C, B, immersed in the water, will have its immersed part, C B, reflected on the surface, as in a mirror, but with a brilliancy far surpassing that which can be obtained from quicksilver, or from the most highly-polished metals, by any means whatever.
OPTICS OF A SOAP BUBBLE.
If a soap-bubble be blown up, and set under a glass, so that the motion of air may not affect it, as the water glides down the sides and the top grows thinner, several colors will successively appear at the top, and spread themselves from thence down the sides of the bubble, till they vanish in the same order in which they appear. At length, a black spot appears at the top, and spreads till the bubble bursts.[7]
THE KALEIDOSCOPE.
If any object be placed between two plane mirrors, inclined towards each other at an angle of thirty degrees, three several images will be perceived in the circumference of a circle. On this principle is formed the kaleidoscope, invented by Sir David Brewster, and by means of which the reflected images viewed from a particular point exhibit symmetrical figures, under an infinite arrangement of beautiful forms and colors. The kaleidoscope may be bought at any toy-shop, but it is requisite that every young person should be able to construct one for himself. He must, therefore, procure a tube of tin or paper, of about ten inches in length, and two and a half or three inches in diameter. One end of this should be stopped up with tin or paper, securely fastened, in which is to be made a hole, about the size of a small pea, for the eye to look through. Two pieces of well-silvered looking-glass, B B, are now to be procured; they must be not quite so long as the tube, and they should be placed in the tube lengthwise, at an angle of 60 degrees, meeting together in a point at A, and separating to the points C C, the polished surfaces looking inwards. A circular piece of glass is now to be laid on the top of the edges of the reflectors, B B; which, by their not being quite so long as the tube, will allow room for its falling in, and it will be supported by the edges of the tube, which may be slightly bent over, to prevent the glass from falling out. This having been done, now proceed to make the "cap" of the instrument. A rim of tin or pasteboard must be cut, so as to fit over the glass end of the tube; and in this, on the outer side, a piece of ground glass must be fastened, so that the whole may fit on the tube like the lid of a pill-box. Then, before putting it on, obtain some small pieces of broken glass of various colors, beads, little strips of wire, or any other object, and place them in the cap; and by passing it over the end, so that the broken glass, &c. has free motion, the instrument is complete. To use it, apply the eye to the small hole, and, on turning it, the most beautiful forms will appear, in the most wonderful combinations.
The following curious calculation has been made of the number of changes this instrument will admit of. Supposing it to contain 20 small pieces of glass, and that you make 10 changes in a minute, it will take an inconceivable space of time, _i. e._ 462,880,899,576 years, and 360 days, to go through the immense number of changes of which it is capable.
SIMPLE SOLAR MICROSCOPE.
Having made a circular hole in a window-shutter, about three inches in diameter, place in it a glass lens of about twelve inches focal distance. To the inside of the hole adapt a tube, having at a small distance from the lens a slit, capable of receiving one or two very thin plates of glass, to which the object to be viewed must be affixed by means of a little gum water exceedingly transparent. Into this tube fit another, furnished at its extremity with a lens of half an inch focal distance. Place a mirror before the hole of the window-shutter on the outside, in such a manner as to throw the light of the sun into the tube, and you will have a solar magic lantern.