The Boy's Playbook of Science Part 34

There are other and more refined means of working out the same fact, but for a rough approximation to the truth, the plan already described will answer very fairly.

A most amusing effect can be produced on the principle that every light casts its own shadow, called the "dance of death," or the "dance of the witches;" either of these agreeable subjects are drawn, and the outlines cut out of a sheet of cardboard. If a wet sheet is stretched or hung on one side of a pair of folding doors partly open, and between which the cardboard is tacked up, and the s.p.a.ce left at the top and bottom closed with a dark cloth, directly the room before the sheet is darkened and a lighted candle held behind the figure cut out in the cardboard, one shadow or image is thrown upon the sheet, and these shadows may be increased according to the number of candles used, and if they are held by two or three persons, and moved up and down, or sideways, the shadows follow the direction of the candles, and present the appearance of a dance. (Fig. 256.)

[Page 268]

[Ill.u.s.tration: Fig. 256. "Before the curtain."]

[Ill.u.s.tration: Fig. 257. "Behind the curtain."]

[Page 269]

Another very comic effect of shadow is that called "jumping up to the ceiling," and when carried out on a large scale by the author on an enormous sheet suspended in the centre transept of the Crystal Palace, Sydenham, it had a most laughable effect, and caused the greatest amus.e.m.e.nt to the children of all ages. (Fig. 258.)

[Ill.u.s.tration: Fig. 258. The laughable effect of the shadows at the Crystal Palace.]

This very telling result is produced by placing an oxy-hydrogen light some feet behind a large sheet, and of course if any one pa.s.ses between the two a shadow of the individual is cast upon the sheet, then by walking towards the light the figure diminishes in size, and by jumping over it the shadow appears to go up to the ceiling, and to come down when the jump is made in the opposite direction over the light and towards the sheet. The _rationale_ of this experiment is very simple, and is [Page 270] another proof of the distribution of light from a luminous source being in every direction. By jumping over the light the radii projected from the candle over the sheet are crossed, and the shadow rises or falls as the figure pa.s.ses upwards or downward. (Fig.

259.)

[Ill.u.s.tration: Fig. 259. The rays of light marked A B C D E proceeding from a lighted candle or oxy-hydrogen light. The arrow pointing to the right shows how these rays are crossed in jumping up to the ceiling; and the second arrow, pointing to the left, shows the reverse.]

A beam of light is defined to be a collection of rays, and it is a convenient definition, because it prevents confusion to speak only of one ray in attempting to explain how light is disposed of under peculiar circ.u.mstances.

The smallest portion of light which it is supposed can be separated is therefore called a ray, and it will pa.s.s through any medium of the same density in a perfectly straight line; but if it pa.s.ses out of that medium into another of a different density, or into any other solid, fluid, or gaseous matter, it may be disposed of in four different ways, being either reflected, refracted, polarized, or absorbed.

The reflection of light is the first property that will be considered, and it will be found that every substance in nature possesses in a greater or lesser degree the power of throwing off the rays of light which fall upon them. Thus if we go into a room perfectly darkened, containing every kind of work produced by nature or art, such as flowers, birds, boxes of insects, rich carpets, hangings, pictures, statuary, jewellery, &c., they cannot excite any pleasure because they are invisible, but directly a lighted lamp is brought into the chamber, then the rays fall upon all the surrounding objects, and being reflected from their surfaces enter the eye, and there produce the phenomena of vision.

This connexion between luminous and non-luminous bodies becomes very apparent when we consider that the sun would appear only as an intense light in a dark background, if the earth was not surrounded with the various strata of air, in which are placed clouds and vapours that collectively reflect and scatter the light, so as to cause it to be endurable to vision. It is when the sky is very clear during July or August that the heat becomes so intense, directly clouds begin to form and float about, the heat is then moderated.

Many years ago, Baron Alexander Funk, visiting some silver mines in Sweden, observed, that in a clear day it was as dark as pitch underground in the eye of the pit at sixty or seventy fathoms deep; whereas, on a cloudy or rainy day he could even see to read at 106 fathoms deep. Inquiring of the miners, he was informed that this is always the case, and [Page 271] reflecting upon it he imagined very properly that it arose from this circ.u.mstance--that when the atmosphere is full of clouds, light is reflected from them into the pit in all directions, so that thereby a considerable proportion of the rays are reflected perpendicularly upon the earth; whereas when the atmosphere is clear there are no opaque bodies to reflect the light in this manner, at least, in a sufficient quant.i.ty, and rays from the sun itself can never fall perpendicularly in Sweden. The use of reflecting surfaces has now become quite common in all crowded cities, and especially in London, where even the rays of light are too few to be lost, and flat or corrugated mirrors are placed at various angles, either to throw the light from the outside on the white-washed ceiling within, and thus obtain a better diffused light through the apartment, or it is reflected bodily to some back room, or rather dark brick box, where perhaps for half a century candles have been required at an early hour in the afternoon. The brilliant cut in diamonds is such an arrangement of the posterior facets, or cut faces of the jewel, that all light reaching them shall be thrown back and reflected, and thus impart an extraordinary brilliancy to the gem.

The intense glare of snow in the Alpine regions has long been noticed, and the reflected light is so powerful, that philosophers were even disposed to believe that snow possessed a natural or inherent luminosity, and gave out its own light. Mr. Boyle, however, disproved this notion by placing a quant.i.ty of snow in a room from which all foreign light was excluded, and neither he nor his companion could observe that any light was emitted, although, on the principle of momentary phosph.o.r.escence, it is quite possible to conceive that if the snow was suddenly brought into a darkened room after exposure to the rays of the sun, that it would give out for a few seconds a perceptible light. In trying such an experiment, one person should expose the snow to the sun, and bring it into a perfectly darkened room to a second person, whose eyes would be ready to receive the faintest impression of light, and if any phosph.o.r.escence existed, it must be apparent.

The property of reflection is also ill.u.s.trated on a grand scale in the illumination of our satellite, the moon, and the various planetary bodies which s.h.i.+ne by light reflected from the sun, and have no inherent self-luminosity. Aristotle was well aware that it is the reflection of light from the atmosphere which prevents total darkness after the sun sets, and in places where the sun's rays do not actually fall during the daytime. He was also of opinion that rainbows, halos, and mock suns, were all occasioned by the reflection of the sunbeams in different circ.u.mstances, by which an imperfect image of the sun was produced, the colour only being exhibited, but not the proper figure.

The image, Aristotle says, is not single, as in a mirror, for each drop of rain is too small to reflect a visible image, but the conjunction of all the images is visible. Aristotle ascribed all these effects to the _reflection_ of light, and it will be noticed when we come to the consideration of the refraction of light, that of course his views must be seriously modified.

[Page 272]

The reflection of light is affected rather by the condition of the surface than the whole body of a substance, as a piece of coal may be covered with gold or silver leaf and caused to s.h.i.+ne, whilst the brightest mirror is dimmed by the thinnest film of moisture.

From whatever surface light is reflected, it always takes place in obedience to two fixed laws.

First. _The incident and reflected rays always lie in the same plane._

Second. _The angle of incidence is equal to the angle of reflection._

With a single jointed two-foot rule, both of these laws are easily ill.u.s.trated. The rule may be held in the hand, and one end being marked with a piece of white paper may be called the incident ray, _i.e._, the ray that falls upon the surface; and the other is the reflected ray, the one cast off or thrown back. A perpendicular is raised by holding a stick upright at the joint. (Fig. 260.)

[Ill.u.s.tration: Fig. 260. A D. A two foot rule; the end A may be termed the incident ray, and the end D the reflected ray. S. The stick held perpendicularly. The angle A B C is equal to the angle D E F, and the whole may be moved in any direction or plane, either horizontal or perpendicular, G G. The reflecting surface.]

One of the most simple and pleasing delusions produced by the reflection of light, is that afforded by cutting through the outline of a vase, or statuette, or flower, drawn on cardboard, and if certain points are left attached, so that the design may not fall out, all the effect of solidity is given by bending back the edges of the cardboard, so that the light [Page 273] from a candle placed behind it, may be reflected from the back edge of one cardboard on to the design, which is bent back. The light reflected from one surface on to the other, imparts a peculiarly soft and marble-like appearance, and when the design is well drawn and cut, and placed in a good position, the illusion is very perfect, and it appears like a solid form instead of a mere design cut out of cardboard. (Fig. 261.)

[Ill.u.s.tration: Fig. 261. Cardboard design in frame, cut and bent back.

The lighted candle is behind.]

The leaf at the side of the above picture is intended to give an idea of the mode of cutting out the designs, and in this case the leaf would be cut and bent back, and a small attachment slip of cardboard left to prevent it falling out.

The cardboard design is always bent toward the light, which is placed behind it. As a good ill.u.s.tration of the importance of reflected light and its connexion with luminous bodies, a beam of light from the oxy-hydrogen lantern may be allowed to pa.s.s above the surface of a table, when it will be noticed that the latter is lighted up only when the beam is reflected downward by a sheet of white paper.

By reference to the two laws of reflection already explained, it is easy to trace out on paper, with the help of compa.s.ses and rule, the effect of plane, concave, and convex surfaces on parallel, diverging, or converging rays of light, and it may perhaps a.s.sist the memory if it is remembered that a _plane_ surface means one that is flat on both sides, such as a looking-gla.s.s: a _convex_ surface is represented by the outside of a watch-gla.s.s; a _concave_ surface, by the inside of a watch-gla.s.s; parallel [Page 274] rays are like the straight lines in a copy-book; diverging and converging rays, are like the sticks of a fan spread out as the sticks separate or diverge; the sticks of the fan come together, or converge at the handle.

The reflection of rays from a plane surface may be better understood by reference to the annexed diagram. (Fig. 262.)

[Ill.u.s.tration: Fig. 262. A I, A K. Two diverging rays incident on the plane surface, D. A D is perpendicular, and is reflected back in the same direction. A I is divergent, and is thrown off at I L. The incident and reflected rays forming equal angles, as proved by the perpendicular, H. Any image reflected in a plane mirror appears as far behind it as the object is before it, and the dotted lines meeting at G show the apparent position of the reflected image behind the gla.s.s, as seen at G. The same fact is also shown in the second diagram, where the reflected picture, I M, appears at the same distance behind the surface of the mirror as the object, A B, is before it.]

By the proper arrangement of _plane_ mirrors, a number of amusing delusions may be produced, one of which is sometimes to be met with in the streets, and is called "the art of looking through a four-inch deal board." The spectator is first requested to look into a tube, through which he sees whatever may be pa.s.sing the instrument at the time; the operator then places a deal board across the middle of the tube, which is cut away for that purpose, and to the astonishment of the juveniles the view is not impaired, and the spectator still fancies he is looking through a straight tube; this however is not the case, as the deception is entirely carried out by reflection, and is explained in the next cut.

(Fig. 263.)

[Page 275]

[Ill.u.s.tration: Fig. 263. A A A A. The apertures through which the spectator first looks. B. The piece of wood, four inches thick. C, D, E, F, are four pieces of looking-gla.s.s, so placed that rays of light entering at one end of the tube are reflected round to the other where the eye of the observer is placed.]

During the siege of Sebastopol numbers of our best artillerymen were continually picked off by the enemy's rifles, as well as by cannon shot, and in order to put a stop to the foolhardiness and incautiousness of the men, a very ingenious contrivance was invented by the Rev. Wm.

Taylor, the coadjutor of Mr. Denison in constructing the first "Big Ben"

bell. It was called the reflecting spy-gla.s.s, and by its simple construction rendered the exposure of the sailors and soldiers, who would look over the parapet or other parts of the works to observe the effect of their shot, perfectly unnecessary; whilst another form was constructed for the purpose of allowing the gunner to "lay" or aim his gun in safety. The instruments were shown to Lord Panmure, who was so convinced of the importance of the invention, that he immediately commissioned the Rev. Wm. Taylor to have a number of these telescopes constructed; and if the siege had not terminated just at the time the invention was to have been used, no doubt a great saving of the valuable lives of the skilled artillerymen would have been effected in the allied armies. The principle of the reflecting spy-gla.s.s may be comprehended by reference to the next cut. (Fig. 264.)

[Ill.u.s.tration: Fig. 264. A picture of enemy's battery is supposed to be on the mirror, A, whence it is reflected to B, and from that to the artilleryman at C.]

By placing two mirrors at an angle of 45, the reflected image of a person gazing into one is thrown into the other, and of course the effect is somewhat startling when a death's head and cross bones, or other [Page 276] cheerful subject, is introduced opposite one mirror, whilst some person who is unacquainted with the delusion is looking into the other. Two adjoining rooms might have their looking-gla.s.ses arranged in that manner, provided there is a pa.s.sage running behind them. (Fig.

265.)

[Ill.u.s.tration: Fig. 265. A. A mirror at an angle of 45 degrees. The arrows show the direction of the reflected image. B. The second mirror, also at an angle of 45 degrees; the face of the person looking in at A is reflected at B. C is the part.i.tion between the rooms.]

One of the most startling effects that can be displayed to persons ignorant of the common laws of the reflection of light, is called the "magic mirror," and is described by Sir Walter Scott in his graphic story of that name. The apparatus for the purpose must be well planned and fixed in a proper room for that purpose, and if carefully conducted, may surprise even the learned. A long and somewhat narrow room should be hung with black cloth, and at one end may be placed a large mirror, so arranged that it will turn on hinges like a door. The magician's circle may be placed at the other end of the chamber in which the spectators must be rigidly confined, and there is very little doubt that the arrangement about to be described was formerly used by clever astrologers who pretended to look into the future, and to hold communication with the supernatural powers. The credulity of the persons who consulted these "wise men," is not surprising when we consider the ignorance of the public generally of common physical laws, and of the wonders that may be worked without the a.s.sistance of the "evil one;"

moreover, the initiated took great care to conceal the machinery of their mysteries, never imparting the illusive tricks even to their most faithful dependents except under solemn oaths of secrecy, because they derived in many cases considerable profit by their pretended conjurations and juggling tricks, and therefore were interested in keeping the outer world in ignorance. The wizards were always careful to impress those who came to consult them with the awful nature of the incantations they were about to perform, and with such a powerful auxiliary as [Page 277] fear, and a well-darkened room, they diverted the thoughts of the more curious, and prevented them watching the proceedings too closely. Theatrical effects were not disdained, such as suppressed and dismal groans, sham thunder, and the wizard usually heightened his own inspiring personal appearance by wearing of course a long beard and flowing robe trimmed with hieroglyphics, and with the a.s.sistance of a ponderous volume full of cabalistic signs, a few skulls and cross bones, an hour-gla.s.s, a pair of drawn swords, a black cat, a charcoal fire, and sundry drugs to throw into it, a very tolerable collection of imps, familiars, and demons, might be expected to attend without the modern practice of spirit-rapping. As before stated, the delusion must be carefully conducted, and a confederate is necessary in order to use the phantasmagoria, or magic lantern. The slides of course were painted to suit the fortune to be unfolded--an easy road to riches for the gentlemen, a tale of love, ending in matrimony, for the ladies.

The spectators being placed in the magic circle, are directed to look into the mirror; they may even be ordered singly to fetch a skull off the mantel-shelf beside the mirror, and whilst doing so to look full into the mirror, and then return to the circle. Absolute silence is enjoined, and soft music is now heard; the darkened room is lit up for the moment by a little yellow or green fire thrown on to the charcoal fire, and now looking into the mirror, it no longer reflects surrounding objects, but a picture, at first small and faint, and then gradually becoming large and clearer, is apparent. The picture is made visible by the confederate gently drawing the mirror from its position parallel with the frame to an angle of 45 degrees, and then throwing on from the side a picture from a magic-lantern. The picture is small and indistinct whilst the confederate holds it near the mirror and out of focus, but as he moves backwards and focuses the lenses, the picture gradually increases in size, and the reflecting angles having been well planned beforehand, only those in the circle will be able to see the picture, and great fun may be elicited from the magic mirror by pretending to tell the future fate of a very slim person, and introducing him by a succession of pictures which gradually a.s.sume a John Bull rotundity of figure, surrounded by dozens of children; whilst to young ladies who are engaged, a provoking picture of an old maid may be introduced; indeed, there is no end to the innocent fun that may be extracted from the magic mirror, and the whole plan of the delusion may be better understood by reference to the next picture. (Fig. 266.)

Monsieur Salverte very properly remarks that "man is credulous from his cradle to his tomb; but the disposition springs from an honourable principle, the consequences of which precipitate him into many errors and misfortunes.... The novelty of objects, and the difficulty of referring them to known objects, will not shock the credulity of unsophisticated men. There are some additional sensations which he receives without discussion, and their singularity is perhaps a charm which causes him to receive them with greater pleasure. _Man almost always_ loves and seeks the marvellous. Is this taste natural?

[Page 278]

[Ill.u.s.tration: The magic mirror.]

[Ill.u.s.tration: Fig. 266. Plan of room. A A. The frame of the looking-gla.s.s. A B. Mirror put back to an angle of 45 degrees. C. The confederate who manages the lantern and shuts the gla.s.s to the frame after each fortune is told. D. The magic circle, to which the rays are reflected.]

[Page 279]