Part 5
The camera obscura used in a public exhibition is a large wooden box stained black in the inside, and capable of containing from one to eight persons. It contains a sliding piece, having a sloping mirror and a double convex lens which may with the mirror 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, 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.
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 colors 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 passes through a great plano-convex lens 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 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 colors, shading with bister also mixed with varnish. The transparent colors 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, etc., 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 colors, the light shines through them, and no light can come upon the screen except that which passes through the figure.
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 marvelous ways, but in none more striking than in raising an apparent specter. Let an open box, 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, to which a cord and pulley should be attached, the end of the cord coming to the back part of the box. 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, and in its lower part 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 place a chafing-dish, upon which put some burning charcoal. Now light the lamp 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 toward 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, etc.; 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 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, 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 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, which must be very powerful, should be so placed that none of it may reach the opening.
TO SHOW THAT RAYS OF LIGHT DO NOT OBSTRUCT EACH OTHER.
Make a small hole in a sheet of pasteboard, and placing it upright before three candles, placed closely together, it will be found that the images of all the candle flames will be formed separately on a piece of paper, 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 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 appeared. At length a black spot appears at the top, and spreads till the bubble bursts.
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 novelty store, 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 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, and separating to an angle wide enough to insert the third piece; 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; 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, etc., 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 where 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 lantern.
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 movable 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 movable 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.
THE MICROSCOPE.
At any time of the year or hour of the day there are few pursuits more interesting, and at the same time instructive, than the study of Nature by means of the microscope.
All of us must admire the more than awful grandeur of that universe whereof we form so infinitesimal a part, wherein the stars are scattered as the sand on the sea-shore, and every star a sun, the center of a system of orbs too distant for the eye of man to perceive. Looking at our nearest planet, and observing on her face vast mountain-chains, ravines into which the light of the sun can never penetrate, and volcanoes whose craters are so wide that they would take in the whole of New York, the whole of Philadelphia, and all the country between them, we can judge by analogy of the unseen wonders which must exist in the world beyond our ken.
But to him who can read Nature rightly, the microscope is a teacher as grand as its sister instrument, and the awful magnificence of Nature is as evident in a midge’s wing as in the more patent glories of the sun, moon, and stars. In the following pages we hope to put the readers of this book in the way to read their microscope rightly--possibly to make it--and to show that much can be done with small means when “there’s a will,” and to indicate to them that objects of no small interest can be found without stirring from the room in which we sit, or even from the table on which our microscope is placed.
Some of our readers may say, when they read the heading of this paper, that they should like a microscope very much, but that they have no money to buy it, and that their parents cannot afford one.
This is just the feeling which we used to have when a boy, for in those day microscopes were microscopes indeed, and you had your choice between a little instrument, with a series of brass cups, having glasses in them, which magnified slightly but defined clearly, or a great composition of brass and iron, looking like a rocket-tube, with an eye-piece at one end and a glass shot at the other. It was very costly, very imposing, and magnified very highly; but it strained the eyes painfully, had no defining capacities, and made all the objects look as if they were seen through a thick fog. Practically, therefore, the former was the only instrument that was available.
A still more useful instrument, however, was that which can always be obtained for a dollar or so, and which is now made wonderfully cheap and wonderfully good; we mean the double or treble pocket-lens. So we say, if you cannot afford a really good microscope, do not waste your money upon inferior and pretentious instruments, but get a sound pocket-lens.
It has a thousand advantages. It is portable, and is even more useful in the fields than in the house. It defines very clearly, and needs little trouble in manipulation. We need not say how difficult is the task of getting a complicated instrument to define properly, how impossible with a bad one. The object and the glass can be held in any light,--a matter of no small consideration when examining anything new, and trying to make out its structure. It is not easily put out of order, and if treated with the most ordinary care, will last for a lifetime.
You can push it under water, and it will magnify as well as in the air; and if you are wandering on the river-side, you can lie down on the bank, dip the upper part of your head in water, together with the glass, and watch carefully the sub-aquatic objects without removing them. The water will not hurt the eye in the least, though a non-swimmer may perhaps find a little difficulty in his first attempt. It makes a good burning-glass, should fire be needed, and no other means of procuring a spark be at hand. It can be used so as to show the principle of a camera obscura, and to illustrate the manner in which photographic portraits are taken. It can be made into an admirable dissecting microscope, and needs scarcely any practice in the manipulation. These are some of its advantages, and there are many others which need not be mentioned.
Even if you should be able to procure a good microscope, get a pocket-lens as well, for you will want them both, and we may say that the most practiced microscopists, and those who are possessors of the most elaborate instruments, are the very men who are the most certain to have a pocket-lens about them, and to use it most frequently. Practice well with the pocket-lens before you meddle with the compound microscope. You will waste no time, but will rapidly gain by it; for you will be learning the rudiments of a new science, and laying a solid foundation on which to build.
One or two practical remarks on the proper handling of the pocket-lens may be of use. Do not always employ the same eye in looking through the lens, but use the eyes alternately. There is always a temptation to employ the same eye, which receives a kind of training in vision; but it is a temptation always to be resisted. With some persons the right eye is most in favor, and with others the left; and when the favorite eye gets all the work, it too frequently suffers. Whether you look with the right or the left eye, _keep both eyes open_.
At first the beginner will find a little difficulty in restricting his vision to one eye while the other remains open, just as a beginner on the piano-forte feels himself puzzled when he tries to make his right hand go one way and his left hand another; but in either case a little practice and plenty of perseverance are sure to overcome all obstacles, and in a wonderfully short time the difficulty will not only be overcome, but forgotten.