Chapter XXI: Light, Optics, and Optical Instruments (1)

"To gild refined gold, to paint the lily,
To throw a perfume on the violet,
To smooth the ice, or add another hue
Unto the rainbow, or with taper light
To seek the beauteous eye of heaven to garnish,
Is wasteful and ridiculous excess."

Perfection admits of no addition, and it is just this feeling that might check the most eloquent speaker or brilliant writer who attempted to offer in appropriate language, the praises due to that first great creation of the Almighty, when the Spirit of God moved upon the face of the waters and said, "Let there be light." If any poet might be permitted to laud and glorify this transcendant gift, it should be the inspired Milton; who having enjoyed the blessing of light, and witnessed the varied and beautiful phenomena that accompany it, could, when afflicted by blindness, speak rapturously of its creation, in those sublime strains beginning with--

[Page 256]

"'Let there be light,' said God, and forthwith light
Ethereal, first of things, quintessence pure,
Sprung from the deep: and from her native east
To journey through the airy gloom began,
Sphered in a radiant cloud, for yet the sun
Was not; she in a cloudy tabernacle
Sojourn'd the while. God saw the light was good,
And light from darkness by the hemisphere
Divided: light the day, and darkness night,
He named."

There cannot be a more glorious theme for the poet, than the vast utility of light, or a more sublime spectacle, than the varied and beautiful phenomena that accompany it. Ever since the divine command went forth, has the sun continued to shine, and to remain, "till time shall be no more," the great source of light to the world, to be the means of disclosing to the eye of man all the beautiful and varied hues of the organic and inorganic world. By the help of light we enjoy the prismatic colours of the rainbow, the lovely and ever changing and ever varied tints of the forest trees, the flowers, the birds, and the insects; the different forms of the clouds, the lovely blue sky, the refreshing green fields; or even the graceful adornment of "the fair," their beautiful dresses of exquisite patterns and colours. Light works insensibly, and at all seasons, in promoting marvellous chemical changes, and is now fairly engaged and used for man's industrial purposes, in the pleasing art of photography; just as heat, electricity, and magnetism, (all imponderable and invisible agents,) are employed usefully in other ways.

The sources from whence light is derived are six in number. The first is the sun, overwhelming us with its size, and destroying life, sometimes, with his intense heat and light, when the piercing rays are not obstructed by the friendly clouds and vapours, which temper and mitigate their intensity, and prevent the too frequent recurrence of that quick and dire enemy to man, the _coup de soleil_.

The body of the sun is supposed to be a habitable globe like our own, and the heat and light are possibly thrown out from one of the atmospheric strata surrounding it. There are probably three of these strata, the one believed to envelope the body of the sun, and to be directly in contact with it, is called the _cloudy stratum_; next to, and above this, is the luminous stratum, and this is supposed to be the source of heat and light; the third and last envelope is of a transparent gaseous nature. These ideas have originated from astronomers who have carefully watched the sun and discovered the presence of certain black spots called _Maculæ_, which vary in diameter from a few hundreds of miles to 40 or 50,000 miles and upwards. There is also a greyish shade surrounding the black spots called the _Penumbra_, and likewise other spots of a more luminous character termed _Faculæ_; indeed the whole disc of the sun has a mottled appearance, and is stippled over with minute shady dots. The cause of this is explained by supposing that these various spots represent openings or breaks in the atmospheric strata, through which the black body of the sun is apparent or other portions of the three strata, just as if a black ball was covered with red, then with yellow, and finally with blue silk: on cutting through the blue the yellow is apparent; by snipping out pieces of the blue and yellow, the red becomes visible; and by slicing away a portion of the three silk coverings the black ball at last comes into view. On a similar principle it is [Page 257]supposed that the variety of spots and eruptions on the sun's face or disc may be explained. The evolution of light is not, however, confined to the sun, and it emanates freely from terrestrial matter by mechanical action, either by friction, or in some cases by mere percussion. Thus the axles of railway carriages soon become red hot by friction if the oil holes are stopped up; indeed hot axles are very frequent in railway travelling, and when this happens, a strong smell of burning oil is apparent, and flames come out of the axle box. The knife-grinder offers a familiar example of the production of light by the attrition of iron or steel against his dry grindstone.

The same result on a much grander scale is produced by the apparatus invented by the late Jacob Perkins; the combustion of steel ensues under the action, viz., the friction of a soft iron disc revolving with great velocity against a file or other convenient piece of hardened steel. (Fig. 247.)

The stand has a disc of soft iron fixed upon an axis, which revolves on two anti-friction wheels of brass. The disc, by means of a belt worked over a wheel immediately below it, is made to perform 5000 revolutions per minute. If the hardest file is pressed against the edge of the revolving disc, the velocity of the latter produces sufficient heat by the great friction to melt that portion of the file which is brought in contact with it, whilst some particles of the file are torn away with violence, and being [Page 258] projected into the air, burn with that beautiful effect so peculiar to steel. If the experiment is performed in a darkened room, the periphery of the revolving disc will be observed to have attained a luminous red heat. Thirty years ago every house was provided with a "tinder-box" and matches to "strike a light." Since the advent of prometheans and lucifers, the flint and steel, the tinder, and the matches dipped in sulphur, have all disappeared, and now the box might be deposited in any antiquarian museum under the portrait of Guy Fawkes, and labelled, "an instrument for procuring a light, extensively used in the early part of the nineteenth century." (Fig. 248.)

The rubbing of a piece of wood (hardened by fire, and cut to a point) against another and softer kind, has been used from time immemorial by savage nations to evoke heat and light; the wood is revolved in the fashion of a drill with unerring dexterity by the hands of the savage, and being surrounded with light chips, and gently aided by the breath, the latent fire is by great and incessant labour at last procured. How favourably the modern lucifers compare with these laborious efforts of barbarous tribes! a child may now procure a light with a chemically prepared metal, and great merit is due to that person who first devised a method of mixing together phosphorus and chlorate of potash and so [Page 259] adjusted these dangerous materials that they are as safe as the "old tinder-box," and have now become one of our domestic necessaries. Ignition, or the increase of heat in a solid body, is another source of light, and is well illustrated in the production of illuminating power from the combustion of tallow, oil, wax, camphine or coal gas. The term _ignition_ is derived from the Latin (_ignis_, fire), and is quite distinct, and has a totally different meaning from that of _combustion_. If a glass jar is filled with carbonic acid gas, and a little tray placed in it containing some gun cotton, it will be found impossible to fire the latter with a lighted taper, _i.e._ by combustion (_comburo_, to burn), because the gas extinguishes flame which is dependent on a supply of oxygen; whereas if a copper or other metallic wire is made red hot or ignited, the carbonic acid has no effect upon the heat, and the red hot wire being passed through the gas, the gun cotton is immediately fired.

Flame consists of three parts--viz., of an outer film, which comes directly in contact with the air, and has little or no luminosity; also of a second film, where carbon is deposited, and, first by _ignition_, and finally by combustion, produces the light; and thirdly, of an interior space containing unburnt gas, which is, as it were, waiting its turn to reach the external air, and to be consumed in the ordinary manner. (Fig. 249.)

Chemical action and electricity have been so frequently mentioned in this work as a source of heat and light, that it will be unnecessary to do more than to mention them here, whilst phosphorescence (the sixth source of light) in dead and living matter, a spontaneous production of light, is well known and exemplified in the "glow-worm," the "fire-fly," the luminosity of the water of the ocean, or the decomposing remains of certain fish, and even of human bodies. Phosphorescence is still more curiously exemplified by holding a sheet of white paper, a calcined oyster-shell, or even the hand, in the sun's rays, and then retiring quickly to a darkened room, when they appear to be luminous, and visible even after the light has ceased to fall upon them.

For the purpose of examining the temporary phosphorescence of various bodies, M. Becquerel has invented a most ingenious instrument, called the "phosphorescope." It [Page 260] consists of a cylinder of wood one inch in diameter and seven inches long, placed in the angle of a black box with the electric lamp inside, so that three-fourths of the cylinder are visible outside, and the remaining fourth exposed to the interior electric light.

By means of proper wheels the cylinder, covered with any substance (such as Becquerel's phosphori), is made to revolve 300 times in a second, and by using this or a lesser velocity, the various phosphori are first exposed to a powerful light and then brought in view of the spectator outside the box.

It is understood that light is produced by an emanation of rays from a luminous body. If a stone is thrown from the hand, an arrow shot from a bow, or a ball from a cannon, we perfectly understand how either of them may be propelled a certain distance, and why they may travel through space; but when we hear that light travels from the sun, which is ninety-five millions of miles away from the earth, in about seven minutes and a half, it is interesting to know what is the kind of force that propels the light through that vast distance, and also what is supposed to be the nature of the light itself.

There are two theories by which the nature of light, and its propagation through space, are explained; they are named after the celebrated men who proposed them, as also from the theoretical mechanism of their respective modes of propulsion: thus we have the Newtonian or _corpuscular_ theory of light, and the Huyghenian or _undulatory_ theory; the first named after Sir Isaac Newton, and the second after Huyghens, another most learned mathematician. Many years before Newton made his grand discovery of the composition of light in the year 1672, mathematicians were in favour of the _undulatory_ theory, and it numbered amongst its supporters not only Huyghens, but Descartes, Hook, Malebranche, and other learned men. Mankind has always been glad to follow renowned leaders, it is so much easier, and is in most cases perhaps the better course, to resign individual opinion when more learned men than ourselves not only adopt but insist upon the truth of their theories; and this was the case with the corpuscular theory, which had been written upon systematically and supported by Empedocles, a philosopher of Agrigentum in Sicily, who lived some 444 years before the Christian era, and is said to have been most learned and eloquent; he maintained that light consisted of particles projected from luminous bodies, and that vision was performed both by the effect of these particles on the eye, and by means of a visual influence emitted by the eye itself. In course of time, and at least 2000 years after this theory was advanced, philosophers had gradually rejected the corpuscular theory, until the great Newton, about the middle of the seventeenth century, advanced as a champion to the rescue, and stamping the hypothesis with his approval, at once led away the whole army of philosophers in its favour, so that till about the beginning of the nineteenth century the whole of the phenomena of light were explained upon this hypothesis.

The corpuscular theory, reduced to the briefest definition, supposes light to be really a material agent, and requires the student to believe [Page 261] that this agent consists of particles so inconceivably minute that they could not be weighed, and of course do not gravitate; the corpuscles are supposed to be given out bodily (like sparks of burning steel from a gerb firework) from the sun, the fixed stars, and all luminous bodies; to travel with enormous velocity, and therefore to possess the property of _inertia_; and to excite the sensation of vision by striking bodily upon the expanded nerve, the retina, the quasi-mind of the eye. Dr. Young remarks, "that according to this projectile theory the force employed in the free emission of light must be about a million million times as great as the force of gravity at the earth's surface, and it must either act with equal intensity on all the particles of light, or must impel some of them through a greater space than others, if its action be more powerful, since the velocity is the same in all cases--for example, if the projectile force is weaker with respect to red light than with respect to violet light, it must continue its action on the red rays to a greater distance than on the violet rays. There is no instance in nature besides of a simple projectile moving with a velocity uniform in all cases, whatever may be its cause; and it is extremely difficult to imagine that such an immense force of repulsion can reside in all substances capable of becoming luminous, so that the light of decaying wood, or two pebbles rubbed together, may be projected precisely with the same velocity as the light emitted by iron burning in oxygen gas, or by the reservoir of liquid fire on the surface of the sun." Now one of the most striking circumstances respecting the propagation of light, is the _uniformity_ of its velocity in the same medium. These and other difficulties in the application of the corpuscular theory aroused the attention of the late Dr. Young, and in the year 1801 he again revived and supported the neglected undulatory theory with such great ability that the attention of many learned mathematicians was directed to the subject, and now it may be said that the corpuscular theory is almost, if not entirely, rejected, whilst the undulatory theory is once more, and deservedly, used to explain the theory of light, and its propagation through space. By this hypothesis it is assumed that the whole universe, including the most minute pores of all matter, whether solid, fluid, or gaseous, are filled with a highly elastic rare medium of a most attenuated nature, called _ether_, possessing the property of _inertia_ but not of gravitation. This _ether_ is not light, but light is produced in it by the excitation on the part of luminous bodies of a vibratory motion, similar to the undulation of water that produces waves, or the vibration of air affording sound. Water set in motion produces waves. Air set in motion produces waves of sound. Ether, _i.e._ the theoretical ether pervading all matter, likewise set in motion, produces light. The nature of a vibratory medium is indeed better understood by reference to that which we know possesses the ordinary properties of matter--viz., the air; and by tracing out the analogy between the propagation of sound and light, the difficulties of the undulatory theory very quickly vanish. To illustrate vibration it is only necessary to procure a finger glass, and having supported a little ebony ball attached to a silk thread by a bent brass [Page 262] wire directly over it, so that the ball may touch either the outside or the inside of the glass, attention must be directed to the quiescence of the ball when a violin bow is lightly moved over the edge of the glass without producing sound, and to the contrary effect obtained by so moving and pressing the bow that a sharp sound is emitted, when immediately the little ball is thrown off from the edge, the repulsive action being continued as long as the sound is produced by the vibration of the glass. (Fig. 250.)

Here the vibrations are first set up in the glass, and being communicated to the surrounding air, a sound is produced; if the same experiment could be performed in a vacuum, the glass might be vibrated, but not being surrounded with air, no sound would be produced. This fact is proved by first ringing a bell with proper mechanism fixed under the receiver placed on the air-pump plate; the sound of the bell is audible until the pump is put in motion and the receiver gradually exhausted, when the ringing noise becomes fainter and fainter, until it is perfectly inaudible. This experiment is made more instructive by gradually admitting the air again into the exhausted vessel, and at the same time ringing the bell, when the sound becomes gradually louder, until it attains its full power. The sun and other luminous bodies may be compared to the finger glass, and are supposed to be endowed naturally with a vibratory motion (a sort of perpetual ague), only instead of the air being set in motion, the _ether_ is supposed to be thrown into waves, which travel through space, and convey the impression of light from the luminous object. Another familiar example of an undulatory medium is shown by throwing a stone into a pool of water; the former immediately forces down and displaces a certain number of the particles of the latter, consequently the surrounding molecules of water are heaped up above their level; by the force of gravitation they again descend and throw up another wave, this in subsiding raises another, until the force of the original and loftier [Page 263] wave dies away at the edge of the pool into the faintest ripples. It must however be understood that it is not the particles of water first set in motion that travel and spread out in concentric circles; but the force is propagated by the rising and falling of each separate particle of water as it is disturbed by the momentum of the descending wave before it. When standing at a pier-head, or on a rock against which the sea dashes, it is usual to hear the observer cry out, if the weather is stormy and the waves very high, "Oh! here comes a great wave!" as if the water travelled bodily from the spot where it was first noticed, whereas it is simply the force that travels, and is exerted finally on the water nearest the rock. It is in fact a progressive action, just as the wind sweeps over a wide field of corn, and bends down the ears one after the other, giving them for the time the appearance of waves. The principle of successive action is well shown by placing a number of billiard balls in a row, and touching each other; if the first is struck the motion is communicated through the rest, which remain immovable, whilst the last only flies out of its place. The force travels through all the balls, which simply act as carriers, their motion is limited, and the last only changes its position. Progressive movement is also well displayed by arranging six or eight magnetized needles on points in a row, with all their north poles in one direction. (Fig. 252.)

On approaching the north pole of a bar magnet to the same pole of one [Page 264] end of the series of needles, it is very curious to see them turn in the opposite direction progressively, one after the other, as the repulsive power of the bar magnet gradually operates upon the similar poles in the magnetic needles. The undulations of the waves of water are also perfectly shown by using the apparatus consisting of the trough with the glass bottom and screen above it, as described at page 10. The transmission of vibrations from one place to another is also admirably displayed in Professor Wheatstone's Telephonic Concert (see page picture), where the musical instruments, as at the Polytechnic, were placed by the author in the basement, and the vibration only conducted by wooden rods to the sounding-boards above, so that the music was laid on like gas or water. These vibrations or undulations in air, water, and the theoretical ether, have therefore been called waves of water, waves of sound, and waves of light, just as if three clocks were made of three different metals, the mechanism would remain the same, though the material, or in this case the medium, be different in each.

Any increase in the number of vibrations of the air produces acute, whilst a decrease attends the grave sounds, and when the waves succeed each other not less than sixteen times in a second, the lowest sound is produced. Light and colours are supposed to be due to a similar cause, and in order to produce the red ray, no less than 477 millions of millions of vibrations must occur in a second of time; the orange, 506; yellow, 535; green, 577; blue, 622; indigo, 658; violet, 699; and white light, which is made up of these colours, numbers 541 millions of millions of undulations in a second.

Although light travels with such amazing rapidity, there is of course a certain time occupied in its passage through space--there is no such thing as instantaneity in nature. A certain period of time, however small, must elapse in the performance of any act whatever, and it has been proved by a careful observation of the time at which the eclipses of the satellites of Jupiter are perceived, that light travels at the rate of 192,500 miles per second, and by the aberration of the fixed stars, 191,515, the mean of these two sets of observations would probably afford the correct rate. Such a velocity is, however, somewhat difficult to appreciate, and therefore, to assist our comprehension of their great magnitude, Sir J. Herschel has given some very interesting comparative calculations, and coming from such an authority we can readily believe them to be correct.

"A cannon-ball moving uniformly at its greatest velocity would require seventeen years to reach the sun. Light performs the same distance in about seven minutes and a half.

"The swiftest bird, at its utmost speed, would require nearly three weeks to make the tour of the earth, supposing it could proceed without stopping to take food or rest. Light performs the same distance in less time than is required for a single stroke of its wing."

Dismissing for the present the theory of undulations, it will be necessary to examine the phenomena of light, regarding it as radiant matter, without reference to either of the contending theories.

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Light issues from the sun, passes through millions of miles to the earth, and as it falls upon different substances, a variety of effects are apparent. There is a certain class of bodies which obstruct the passage of the rays of light, and where light is not, a shadow is cast, and the substance producing the shadow is said to be opaque. Wood, stone, the metals, charcoal, are all examples of opacity; whilst glass, talc, and horn allow a certain number of the rays to travel through their particles, and are therefore called transparent. Nature, however, never indulges in sudden extremes, and as no substance is so opaque as not (when reduced in thickness) to allow a certain amount of light to pass through its substance, so, on the other hand, however transparent a body may be, a greater or lesser number of the rays are always stopped, and hence opacity and transparency are regarded as two extremes of a long chain; being connected together by numerous intermediate links, they pass by insensible gradations the one into the other.

If a gold leaf, which is about the one two-hundredth part of an inch in thickness, is fixed on a glass plate and held before a light, a green colour is apparent, the gold appearing like a green, semi-transparent substance. When plates of glass are laid one above the other, and the flame of a candle observed through them, the light decreases enormously as the number of glass-plates are increased. Even in the air a considerable portion of light is intercepted. It has been estimated that of the horizontal sunbeams passing through about two hundred miles of air, one two-thousandth part only reaches us, and that no sensible light can penetrate more than seven hundred feet deep into the sea; consequently, the vast depths discovered in laying the Atlantic telegraph must be in absolute darkness.

Light is thrown out on all sides from a luminous body like the spokes of a cart-wheel, and in the absence of any obstruction, the rays are distributed equally on all sides, diverging like the radii drawn from the centre of a circle. As a natural consequence arising from the divergence of each ray from the other, the intensity of light decreases as the distance from the luminous source increases, and _vice versâ_. Perhaps the best mechanical notion of this law is afforded by an ordinary fan; the point from which the sticks radiate, and where they all meet, may be [Page 266] termed the light; the sticks are the rays proceeding from it. (Fig. 253.)

The fan is held in one hand, and the first finger of the other can be made to touch all the sticks if placed sufficiently near to A; and supposing the sticks are called rays of light, the intensity must be great at that point, because all the rays fall upon it; but if the finger is removed towards the outer edge--viz., to B, it now only touches some three or four sticks; and pursuing the analogy, a very few rays fall upon that point--hence the light has decreased in intensity, or to speak correctly, "Light decreases inversely as the squares of the distance." This law has already been illustrated at page 13; and as an experiment, the rays from the oxy-hydrogen lantern may be permitted to pass out of a square hole (say two inches square), and should be thrown on to a transparent screen divided into squares by dark lines, so that the light at a certain distance illuminates one of them; then it will be found that at twice the distance, four may be illuminated, at three times nine, and so on. (Fig. 254.)

Upon this law is based the use of photometers, or instruments for measuring light, and supposing it was required to estimate roughly the illuminating power of any lamp, as compared with the light of a wax candle six to the pound, the experiment should be conducted in a dark room, from which every other light but that from the lamp and candle under examination must be excluded.

The lamp, with the chimney only, is now placed say twelve feet from the wall, and a stick or rod is placed upright and about two inches from the latter, so that a shadow is cast on the wall; if the candle is now lighted and allowed to burn up properly, two shadows of the stick will be apparent, the one from the lamp being black and distinct, and the other from the candle extremely faint, until it is approached nearer the [Page 267] wall--say to within three feet--when the two shadows may be now equal in blackness. (Fig. 255.) After this is apparent to one or more persons, the distances of the lamp and candle from the wall are carefully measured, and being squared, and the greater divided by the lesser number, the quotient gives the illuminating power. For example:

The lamp was 12 feet from the wall 12 × 12 = 144.
The candle was 3 feet " 3 × 3 = 9.
9) 144
----
16

Therefore the illuminating power of the lamp is equal to 16 wax candles six to the pound.

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 space 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.)

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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 amusement to the children of all ages. (Fig. 258.)

This very telling result is produced by placing an oxy-hydrogen light some feet behind a large sheet, and of course if any one passes 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 passes upwards or downward. (Fig. 259.)

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 circumstances.

The smallest portion of light which it is supposed can be separated is therefore called a ray, and it will pass through any medium of the same density in a perfectly straight line; but if it passes 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 circumstance--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 quantity, 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 quantity 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 phosphorescence, 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 phosphorescence existed, it must be apparent.

The property of reflection is also illustrated on a grand scale in the illumination of our satellite, the moon, and the various planetary bodies which shine 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 circumstances, 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.

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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 shine, 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 illustrated. 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.)

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.)

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 illustration 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 pass 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 compasses and rule, the effect of plane, concave, and convex surfaces on parallel, diverging, or converging rays of light, and it may perhaps assist the memory if it is remembered that a _plane_ surface means one that is flat on both sides, such as a looking-glass: a _convex_ surface is represented by the outside of a watch-glass; a _concave_ surface, by the inside of a watch-glass; 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.)

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 passing 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.)

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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-glass, 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-glass may be comprehended by reference to the next cut. (Fig. 264.)

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-glasses arranged in that manner, provided there is a passage running behind them. (Fig. 265.)

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 assistance 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 assistance of a ponderous volume full of cabalistic signs, a few skulls and cross bones, an hour-glass, 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 assume 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?

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Does it spring from the education which during many ages the human race has received from its first instructors? A vast and novel question, but with which I have nothing to do. It is sufficient to observe that as the lover of the wonderful always prefers the most surprising to the most natural account, this last has been too frequently neglected, and is irrevocably lost. Occasionally, however (and we shall cite more than one instance), simple truth has escaped from the power of oblivion. Credulous man may be deceived once, or more frequently; but his credulity is not a sufficient instrument to govern his whole existence. The wonderful excites only a transient admiration. In 1798, the French _savans_ remarked with surprise how little the spectacle of balloons affected the indolent Egyptian.... But man is led by his passions, and particularly by _hope_ and _fear_."

When parallel rays fall upon a convex mirror, they are scattered and dispersed in all directions, and the image of an object reflected in a convex mirror appears to be very small, being reduced in size because the reflected picture I M is nearer the surface of the mirror than the object A B. No. 1. (Fig. 267.)

Convex mirrors are not employed in any optical deception on a large scale, although some ingenious delusions are producible from cylindrical and conical mirrors, and are thus described by Sir David Brewster:

"Among the ingenious and beautiful deceptions of the seventeenth century, we must enumerate that of the re-formation of distorted pictures by reflection from cylindrical and conical mirrors. In these representations, the original image from which a perfect picture is produced, [Page 280] is often so completely distorted, that the eye cannot trace in it the resemblance to any regular figure, and the greatest degree of wonder is of course excited, whether the original image is concealed or exposed to view. These distorted pictures may be drawn by strict geometrical rules, and I have shown a simple method of executing them. Let M be an accurate cylinder made of tin-plate or of thick pasteboard. Out of the further side of it cut a small aperture, _a b c d_, and out of the nearer side cut a larger one, A B C D (white letters), the size of the picture to be distorted; having perforated the outline of the picture with small holes, place it in the opening A B C D (white letters), so that its surface may be cylindrical; let a candle or a bright luminous object--the smaller the better--be placed at S, as far behind the picture A B C D (white letters) as the eye is afterwards to be placed before it, and the light passing through the small holes will represent on a horizontal plane a distorted image of the picture at A B C D, which, when sketched in outline with a pencil, shaded, and coloured, will be ready for use. If we now substitute a polished cylindrical mirror of the same size in place of M, then the distorted picture, when laid horizontally at A B C D, will be restored to its original state when seen by reflection at A B C D (white letters) in the polished mirror." The effect of a cylindrical mirror on a distorted picture is shown at No. 2, being copied from an old one seen by Sir D. Brewster.

By looking at a reflection of the face in a dish-cover or the common surface of a bright silver spoon or of a silver mug, the latter truly becomes ugly as the image is seen reflected from its surface, and [Page 281] assumes the most absurd form as the mouth is opened or shut, and the face advanced or removed from the silver vessel. (Fig. 269.)

In the writings of the ancients there are to be found certain indications of the results of illusions produced by simple optical arrangements, and the sudden and momentary apparition (from the gloom of perfect darkness) of splendid palaces, delightful gardens, &c., with which-the concurrent voice of antiquity assures us-the eyes of the beholders were frequently dazzled in the mysteries, such as the evocation and actual appearance of departed spirits, the occasional images of their _umbræ_, and of the gods themselves. From a passage in "Pausanias," (Boeotic XXX.), when, speaking of Orpheus, he says there was anciently at Aornos, a place where the dead were evoked, [Greek: _nekuomanteion_], we learn that in those remote ages there were places set apart for the evocation of the dead. Homer relates, in the eleventh book of the "Odyssey," the admission of Ulysses alone into a place of this kind, when his interview with his departed friend was interrupted by some fearful voice, and the hero, apprehending the wrath of Proserpine, withdrew; the priests who managed these deceptive exhibitions no doubt adopted this method of getting rid of their visitor, who might become too inquisitive, and discover the secret of the mysteries.