OPTICS AND OPTICAL AMUSEMENTS.
“‘Seeing is believing,’ so the sages say,
To prove this false, hear me, my friends, I pray,
And very soon you all will be agreeing,
That nought is so deceptive as our seeing.”—Martin.
Optics is the science of light and vision. Concerning the nature of light, two theories are at present very ably maintained by their respective advocates. One is termed the Newtonian theory, and the other the Huygenean. The Newtonian theory considers light to consist of inconceivably small bodies emanating from the sun, or any other luminous body. The Huygenean conceives it to consist in the undulations of a highly elastic and subtle fluid, propagated round luminous centres in spherical waves, like those arising in a placid lake when a stone is dropped into the water.
LIGHT AS AN EFFECT.
Light follows the same laws as gravity, and its intensity or degree decreases as the square of the distance from the luminous body increases. Thus, at the distance of two yards from a candle we shall have four times less light than we should have, were it only one yard from it, and so on in the same proportion.
REFRACTION.
Bodies which suffer the rays of light to pass through them, such as air, water, or glass, are called refracting media. When rays of light enter these, they do not proceed in straight lines, but are said to be refracted, or bent out of their course, as seen in the [drawing]. The ray of light proceeding from B through the glass L G is bent from the point C, instead of passing in the direction of the dotted line. But if the ray F C falls perpendicularly on the glass, there is no refraction, and it proceeds in a direct line to K; hence refraction only takes place when rays fall obliquely or aslant on the media.
THE INVISIBLE COIN MADE VISIBLE.
If a coin be placed in a basin, so that on standing at a certain distance it be just hid from the eye of an observer by the rim or edge of the basin, and then water be poured in by a second person, the first keeping his position; as the water rises the coin will become visible, and will appear to have moved from the side to the middle of the basin.
THE MULTIPLYING GLASS.
The multiplying glass is a semicircular piece of glass cut into facets or distinct surfaces; and in looking through it we have an illustration of the laws of refraction, for if a small object, such as a fly, be placed at D, an eye at E will see as many flies as there are surfaces or facets on the glass.
TRANSPARENT BODIES.
Transparent bodies, such as glass, may be made of such form as to cause all the rays which pass through them from any given point to meet in any other given point beyond them, or which will disperse them from the given point. These are called lenses, and have different names according to their form. 1. Is called the plano-convex lens. 2. Plano-concave. 3. Double convex. 4. Double concave. 5. A meniscus, so called from its resembling the crescent moon.
THE PRISM.
The prism is a triangular solid of glass, and by it the young optician may decompose a ray of light into its primitive and supplementary colours, for a ray of light is of a compound nature. By the prism the ray A is divided into its three primitive colours, blue, red, and yellow; and their four supplementary ones, violet, indigo, green, and orange. The best way to perform this experiment is to cut a small slit in a window-shutter, on which the sun shines at some period of the day, and directly opposite the hole place a prism P; a beam of light in passing through it will then be decomposed, and if let fall upon a sheet of white paper, or against a white wall, the seven colours of the rainbow will be observed.
COMPOSITION OF LIGHT.
The beam of light passing through the prism is decomposed, and the spaces occupied by the colours are in the following proportions:—red, 6; orange, 4; yellow, 7; green, 8; blue, 8; indigo, 6; violet, 11. Now, if you paste a sheet of white paper on a circular piece of board about six inches in diameter, and divide it with a pencil into fifty parts, and paint colours in them in the proportions given above, painting them dark in the centre parts, and gradually fainter at the edges, till they blend with the one adjoining. If the board be then fixed to an axle, and made to revolve quickly, the colours will no longer appear separate and distinct, but becoming gradually less visible they will ultimately appear white, giving this appearance to the whole surface of the paper.
A NATURAL CAMERA OBSCURA.
The human eye is a camera obscura, for on the back of it on the retina every object in a landscape is beautifully depicted in miniature. This may be proved by the
BULLOCK’S EYE EXPERIMENT.
Procure a fresh bullock’s eye from the butcher, and carefully thin the outer coat of it behind: take care not to cut it, for if this should be done the vitreous humour will escape, and the experiment cannot be performed. Having so prepared the eye, if the pupil of it be directed to any bright objects, they will appear distinctly delineated on the back part precisely as objects appear in the instrument we are about to describe. The effect will be heightened if the eye is viewed in a dark room with a small hole in the shutter, but in every case the appearance will be very striking.
THE CAMERA OBSCURA.
This is a very pleasing and instructive optical apparatus, and may be purchased for four or five shillings. But it may be easily made by the young optician. Procure an oblong box, about two feet long, twelve inches wide, and eight high. In one end of this a tube must be fitted containing a lens, and be made to slide backwards and forwards so as to suit the focus. Within the box should be a plain mirror reclining backwards from the tube at an angle of forty-five degrees. At the top of the box is a square of unpolished glass, upon which from beneath the picture will be thrown, and may be seen by raising the lid A. To use the camera place the tube with the lens on it opposite to the object, and having adjusted the focus, the image will be thrown upon the ground-glass as above stated, where it may be easily copied by a pencil or in colours.
The form of a camera obscura used in a public exhibition is as follows:—D D is a large wooden box stained black in the inside, and capable of containing from one to eight persons. A B is a sliding piece, having a sloping mirror C, and a double convex lens F, which may with the mirror C be slid up or down so as to accommodate the lens to near and distant objects. When the rays proceeding from an object without fall upon the mirror, they are reflected upon the lens F, and brought to fall on the bottom of the box, or upon a table placed horizontally to receive them, which may be seen by the spectator whose eye is at E.
THE CAMERA LUCIDA.
This instrument consists of a glass prism, C, D, D, E, having four sides covered. The sides C, D, being exposed to the object to be delineated, rays pass through the glass and fall on the sloping side D, E; from this they are reflected to the top, and finally pass out of the prism to the eye;[11] now from the direction at which the rays enter the eye, it receives them as if coming from an image at A, B, and if a sheet of paper be placed below the instrument, a perfect delineation of the object may be traced with a pencil. This is a very useful instrument to young draughtsmen.
[11] The eye is to be applied to the little circular hole seen on the upper surface.
THE MAGIC LANTERN.
This is one of the most pleasing of all optical instruments, and it is used to produce enlarged pictures of objects, which being painted on a glass in various colours are thrown upon a screen or white sheet placed against the wall of a large room. It consists of a sort of tin-box, within which is a lamp, the light of which (strongly reflected by the reflector T,) passes through a great plano-convex lens E fixed in the front. This strongly illuminates the objects which are painted on the slides or slips of glass, and placed before the lens in an inverted position, and the rays passing through them and the lens F, fall on a sheet, or other white surface, placed to receive the image. The glasses on which the figures are drawn are inverted, in order that the images of them may be erect.
PAINTING THE SLIDES.
The slides containing the objects usually shown in a magic lantern, are to be bought at opticians with the lantern, and can be procured cheaper and better in this way than by any attempt at manufacturing them. Should, however, the young optician wish to make a few slides of objects of particular interest to himself, he may proceed as follows:—
Draw first on paper the figures you wish to paint, lay it on the table, and cover it over with a piece of glass of the above shape; now draw the outlines with a fine camel’s hair pencil in black paint mixed with varnish, and when this is dry, fill up the other parts with the proper colours, shading with bistre also mixed with varnish. The transparent colours are alone to be used in this kind of painting.
TO EXHIBIT THE MAGIC LANTERN.
The room for the exhibition ought to be large, and of an oblong shape. At one end of it suspend a large sheet so as to cover the whole of the wall. The company being all seated, darken the room, and placing the lantern with its tube in the direction of the sheet, introduce one of the slides into the slit, taking care to invert the figures; then adjust the focus of the glasses in the tube by drawing it in or out as required, and a perfect representation of the object will appear.
EFFECTS OF THE MAGIC LANTERN.
Most extraordinary effects may be produced by means of the magic lantern; one of the most effective of which is a
TEMPEST AT SEA.
This is effected by having two slides painted, one with the tempest as approaching on one side, and continuing in intensity till it reaches the other. Another slide has ships painted on it, and while the lantern is in use, that containing the ships is dexterously drawn before the other, and represents ships in the storm.
The effects of sunrise, moonlight, starlight, &c., may be imitated, also by means of double slides, and figures may be introduced sometimes of fearful proportions.
Heads may be made to nod, faces to laugh; eyes may be made to roll, teeth to gnash; crocodiles may be made to swallow tigers; combats may be represented; but one of the most instructive uses of the slides is to make them illustrative of astronomy, and to show the rotation of the seasons, the cause of eclipses, the mountains in the moon, spots on the sun, and the various motions of the planetary bodies, and their satellites.
THE PHANTASMAGORIA.
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 colours, 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 it further from the screen, or bringing it nearer to it. This is a great advantage over the ordinary 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 together the rays of light proceeding from the lenses of each, which produces that confusion of images, in which one view melts as it were into the other, which gradually becomes clear and distinct; the principle being the gradual extinction of one picture, and the production of another.
HOW TO RAISE A GHOST.
The magic lantern, or phantasmagoria, may be used in a number of marvellous ways, but in none more striking than in raising an apparent spectre. Let an open box, A B, about three feet long, a foot and a 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 spectre 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 of light reflected from the glass to pass through them. On the top of the box at 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 spectre 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 centre. On this pasteboard disc, paint in colours 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 ILLUSIONS.
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 coloured 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, and affix it 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.
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 centre 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 centre 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, which must be very powerful, should be so placed that none of it may reach the opening.
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.[12]
[12] 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 proceeded 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 THAT 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.
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 colours 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 appeared. At length, a black spot appears at the top, and spreads till the bubble bursts.[13]
[13] The thinnest substance ever observed is the aqueous film of the soap-bubble previous to bursting; yet it is capable of reflecting a faint image of a candle, or the sun. Hence its thickness must correspond with what Sir Isaac Newton calls the beginning of black, which appears in water at the thickness of the seven hundred and fifty thousandth part of an inch.
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 colours. 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 it lengthways, 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 the 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 colours, 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 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 lanthorn.
The method of employing this arrangement of lenses for microscopic purposes is as follows:—Having darkened the room, and by means of the mirror reflected the sun’s rays on the glasses in a direction parallel to the axis, place some small object between the two moveable plates of glass, or affix it to one of them with very transparent gum-water, and bring it exactly into the axis of the tube; if the moveable tube be then pushed out or drawn in, till the object be a little beyond the focus, it will be seen painted very distinctly on a card, or piece of white paper, held at a proper distance, and will appear to be greatly magnified. A small insect will appear as a large animal, a hair as big as a walking-stick, and the almost invisible eels in paste or vinegar as large as common eels.
ANAMORPHOSES.
This is a very curious optical effect, producing a distorted and grotesque figure from a regular one. The term is derived from two Greek words, signifying a distortion of figure, and by its means many optical puzzles may be produced geometrically.
Take any subject, such as the portrait of a head; divide it vertically and horizontally with parallel lines, of which the outer sides shall form the boundary, A, B, C, D, and the whole shall be equidistant. Then, on a separate piece of paper, or cardboard, prepare a drawing similar to [Fig. 2] by the following means:—
1. Draw a horizontal line, a b, equal to A B, and divide it into as many equal parts as the latter is divided.
2. Let fall a perpendicular line, e v, from the middle of a b, and then draw s v parallel to a b.
3. Both e v and s v may be any length at pleasure, but the longer the first is, and the shorter the other, so will the anamorphoses be more and more deformed. The proportions in our figures are sufficiently different.
4. After having drawn from the point v right lines, v 1, v 2, v 3, v 4, to the divisions of a b, draw the line s b, and through each point where s b intersects the divergent lines draw other horizontal lines parallel to a b. We now have a trapezium, a b c d divided into as many cells as the square in [Fig. 1].
The next step is to fill up all the cells of [Fig. 2] with portions of the device, proportionate to their position in [Fig. 1]. For instance, in [Fig. 1] the nose is in the second vertical division from the left, and in the third and fourth horizontal divisions from the top, and that portion of the face must accordingly be placed in a corresponding part of [Fig. 2].
By these means we procure the anamorphosis seen in [Fig. 2], which when viewed from a particular position, will lose all its distortion, and assume an appearance resembling that in [Fig. 1]. This position lies immediately over the point v, and at a height above it equal to the length of the line s v; and the means of determining it are as follows.
Place the drawing horizontally before a window; take a slip of card, and rest its lower edge on the line s v, the card being accurately vertical; pierce a small hole in the card vertically over the point v, and at a height from it equal to the length of the line s v, then with the eye placed immediately behind the card, look through the orifice at the anamorphosis, and it will be found that as soon as it has become accustomed to the novelty of the experiment, the anamorphosis will lose its distortion, and appear almost exactly like the symmetrical figure.
It would be very difficult, and would require geometrical reasoning of a lengthened kind, to show why this particular form of construction should lead to such results.
THE COSMORAMA.
The principle upon which the cosmorama is formed is so simple, that any person may easily fit up one in a small summer house, &c. Nothing more is necessary than to fix in a hole a double convex lens of about three feet focus, A, and at rather less than this distance a picture, B, is to be hung. To absorb all the rays of light but those necessary for seeing the picture, a squared frame of wood blackened on the inside is placed between the lens and the picture. The picture may be hung in a large box having a light coming in upon it from above, or in a small closet illuminated in the same manner. Should it be wished to show it by candle-light, a lamp, c, may be placed on the top of the wooden frame, and if the light of this be converged by a lens to a moderate radius, it will be more effective.
DISTORTED LANDSCAPES.
Landscapes or other matters may be drawn so as to produce curious optical illusions by the following method. Take a piece of smooth white pasteboard and sketch the design upon it. Prick the outlines in every part with a fine pin or needle, then place the pricked drawing in a perpendicular position, and put a lighted candle behind it. Place before it another piece of pasteboard, and follow with a pencil the lines given by the light, and you have produced a distorted landscape. Now take away the candle and the pricked drawing, and place your eye where the light was, and the drawing will assume the regular form. To get your eye in the proper position, it will be advisable to cut out a piece of card according to the accompanying [pattern], and raising it on its base, B, look through the hole at A, when the object will appear in its proper proportions.
