LIGHT, OPTICS, AND OPTICAL INSTRUMENTS.
[Illustration: Fig. 246. "The moon shines bright:--In such a night as
this."--_The Merchant of Venice._]
"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.)
[Illustration: Fig. 247. Instrument for the combustion of steel.]
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.)
[Illustration: Fig. 248. C. The steel. B. The flint. E. The tinder. D.
The matches of the old-fashioned tinder-box, A.]
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.
[Illustration: Fig. 249. A candle flame. 1. Outer flame. 2. Inner flame,
which is badly supplied with oxygen, and where the carbon is deposited
and _ignited_. 3. The interior, containing unburnt gas.]
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.)
[Illustration: Fig. 250. A. The finger glass. B. The violin bow. C. The
ebony ball. The dotted ball shows how it is repelled during the
vibration of the glass.]
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.)
[Illustration: Fig. 251. Boy throwing stones into water and producing
circular waves.]
[Illustration: Fig. 252. A B. Series of needles arranged as described.
C. The bar magnet, with the north pole N towards the needles. The dotted
lines show the direction gradually assumed by all the needles,
commencing at D.]
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.
[Page 265]
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.
[Illustration: Fig. 253.]
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.)
[Illustration: Fig. 254. Lantern at the three distances from the
transparent screen, which is divided into nine equal squares.]
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.
[Illustration: Fig. 255. A. The lamp. B. The candle. C. The rod throwing
the two shadows, marked D and E, on a white wall or a sheet of paper.]
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.)
[Page 268]
[Illustration: Fig. 256. "Before the curtain."]
[Illustration: 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
amusement to the children of all ages. (Fig. 258.)
[Illustration: 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 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.)
[Illustration: 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
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.
[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 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.)
[Illustration: 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.)
[Illustration: 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 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.)
[Illustration: 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 glass, 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 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.)
[Page 275]
[Illustration: 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-glass, 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-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.)
[Illustration: 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-glasses arranged
in that manner, provided there is a passage running behind them. (Fig.
265.)
[Illustration: 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 partition 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 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?
[Page 278]
[Illustration: The magic mirror.]
[Illustration: Fig. 266. Plan of room. A A. The frame of the
looking-glass. A B. Mirror put back to an angle of 45 degrees. C. The
confederate who manages the lantern and shuts the glass to the frame
after each fortune is told. D. The magic circle, to which the rays are
reflected.]
[Page 279]
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.)
[Illustration: Fig. 267. A B, D H. (No. 2) represent two parallel rays
incident on the convex surface B H, the one (A B) perpendicularly, the
other (D H) obliquely. C is the centre of convexity. H E is the
reflected ray of the oblique incident one, D H; whilst C H I is the
perpendicular.]
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.
[Illustration: Fig. 268.]
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.)
[Illustration: Fig. 269. Distorted image produced by an irregular convex
surface.]
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.
Of all the reflecting surfaces mentioned, none produce more interesting
deceptions than the concave mirror, and there is very little doubt that
silver mirrors of this form were known to the ancients, and employed in
[Page 282] some of their sacred mysteries. Mons. Salverte has
industriously collected in his valuable work the most interesting proofs
of their use, and quotes the following passage of "Damascius," in which
the results obtainable from a concave mirror are clearly apparent. (Fig.
270.)
[Illustration: Fig. 270. The picture of a human face, possibly reflected
from a concave mirror concealed below the floor of the temple; the
opening being hidden by a raised mass of stone, and the worshippers
confined to a certain part of the temple, and not allowed to approach
it.]
He says:--"In a manifestation which ought not to be revealed ... there
appeared on the wall of the temple a mass of light which at first seemed
very remote; it transformed itself in coming nearer into a face
evidently divine and supernatural, of a severe aspect, but mixed with
gentleness, and extremely beautiful. According to the institution of a
mysterious religion, the Alexandrians honoured it as Osiris and Adonis."
Parallel rays thrown upon a concave surface are brought to a focus or
converge, and when an object is seen by reflection from a concave
surface, the representation of it is various, both with regard to its
[Page 283] magnitude and situation, according as the distance of the
object from the reflecting surface is greater or less. (Fig. 271.) When
the object is placed between the _focus_ of parallel rays and the
centre, the image falls on the _opposite_ side of the centre, and is
larger than the object, and in an inverted position. The rays which
proceed from any remote terrestrial object are nearly parallel at the
concave mirror--not strictly so, but come diverging to it in separate
pencils, or, as it were, bundles of rays, from each point of the side of
the object next the mirror; therefore they will not be converged to a
point at the distance of half the radius of the mirror's concavity from
its reflecting surface, but in separate points at a little greater
distance from the concave mirror. The nearer the object is to the
mirror, the further these points will be from it, and an inverted image
of the object will be formed in them, which will seem to hang pendant in
the air, and will be seen by an eye placed beyond it (with regard to the
mirror), in all respects like the object, and as distinct as the object
itself. No. 2. (Fig. 271.)
[Illustration: Fig. 271. No. 1. A B, D H represent two parallel rays
incident on the concave surface B H, whose centre of concavity is C. B F
and H F are the reflected rays meeting each other in F, and A B being
perpendicular to the concave surface, is reflected in a straight line.
No. 2. A B. The object. I M. The image.]
[Illustration: Fig. 272. A B represents the object, S V the reflecting
surface, F its focus of parallel rays, and C its centre. Through A and
B, the extremities of the object, draw the lines C E and C N, which are
perpendicular to the surface, and let A R, A G, be a pencil of rays
flowing from A. These rays proceeding from a point beyond the focus of
parallel rays, will, after reflection, converge towards some point on
the opposite side of the centre, which will fall upon the perpendicular,
B C, produced, but at a greater distance from C than the radiant A from
which they diverged. For the same reason, rays flowing from B will
converge to a point in the perpendicular N C produced, which shall be
further from C than the radiant B, from whence it is evident that the
image I M is larger than the object A B, that it falls on the _contrary_
side of the centre, and that their positions are inverted with respect
to each other.]
[Page 284]
It appears, from a circumstance in the life of Socrates, that the
effects of burning-glasses were known to the ancients; and it is
probable that the Romans employed the concave speculum for the purpose
of lighting the "sacred fire." This is very likely to be true,
considering that the priests who conducted the heathen worship of Osiris
and Adonis were acquainted with the use of concave metallic specula, as
already described at page 282. The effects that can be produced with the
aid of concave mirrors are very impressive, because they are not merely
confined to the reflection of inanimate objects, but life and motion can
be well displayed by them; thus, if a man place himself directly before
a concave mirror, but further from it than its centre of concavity, he
will see an inverted image of himself in the air between him and the
mirror of a less size than himself; and if he hold out his hand towards
the mirror the hand of the image will come out towards his hand and
coincide with it, being of an equal bulk when his hand is in the centre
of concavity, and he will imagine he may shake hands with his image.
(Fig. 273.)
[Illustration: Fig. 273. A concave mirror, showing the appearance of the
inverted and reflected image in the air.]
By using a large concave mirror of about three feet in diameter, the
author was enabled to show all the results to a large audience that were
usually visible to one person only. Whilst experimenting with a concave
mirror, by holding out the hand in the manner described, a bystander
will see nothing of the image, because none of the reflected rays that
form it enter his eyes. This circumstance is well illustrated by placing
a concave mirror opposite the fire, and allowing the image of the flames
projected from it to fall upon a well-polished mahogany table. If the
door of the room opens towards the mirror, and a spectator unacquainted
with the properties of concave mirrors should enter the apartment, the
person would be greatly startled to see flames apparently playing over
the surface of the table, whilst another spectator might enter from
another door and see nothing but a long beam of light, rendered visible
by the floating particles of dust. To give proper effect to this
experiment the concave mirror should be large, and no other light must
illuminate the room except that from the fire.
On the same polished table the appearance of a planet with a revolving
[Page 285] satellite may be prettily shown by darkening the fire with a
screen, and placing a lighted candle before it, which will be reflected
by the concave mirror, and appear on the table as a brilliant star of
light, and the satellite may be represented by the flame of a small wax
taper moved around the large burning candle. The following is the
arrangement used by the author at the Polytechnic Institution for the
purpose of exhibiting the properties of the concave mirror. A lantern
enclosing a very brilliant light, such as the electric or lime light, is
required for the illumination of the objects which are to be projected
on to the screen. The lantern and electric lamp of Duboscq was
preferred, although, of course, any bright light enclosed in a box, with
a plain convex lens to project the beam of light when required, will
answer the purpose. (Fig. 274.)
[Illustration: Fig. 274. A B. Portable screen of light framework,
covered with black calico. C C C C. Square aperture just above the
shelf, D D, upon which the object--viz., a bottle half full of water--is
placed. E. Duboscq lantern to illuminate the object at D D.]
By removing the diaphragm required to project the picture of the
charcoal points on to the screen, a very intense beam of light is
obtained, which may be focussed or concentrated on any opaque object by
another double convex lens, conveniently mounted with a telescope stand,
so that it may be raised or lowered at pleasure. This lens is
independent of the lantern, and may be used or not at the pleasure of
the operator.
The object is now placed on a shelf fixed to the screen, with a square
aperture just above it. The object of the screen is to cut off all
extraneous rays of light reflected from the mirror, or to increase the
sharpness of the outline of the picture of the object. The screen and
object being arranged, and the light thrown on from the lantern, the
next step is to adjust the concave mirror, and by moving it towards the
[Page 286] object, or backwards, as the case requires, a good image,
solid and quasi-stereoscopic, is projected on to the screen. (Fig. 275.)
[Illustration: Fig. 275. A. The concave mirror. B. The lantern. C. The
portable screen, shelf, and object. D. The inverted image of the bottle
filling with water, with the neck downwards, and when thrown on the disc
at D producing a most curious illusion.]
The act of filling the bottle with water, or better still with mercury,
is one of the most singular effects that can be shown; and if all the
apparatus is enclosed in a box, so that the picture on the screen only
is apparent, the illusion of a bottle being filled in an inverted
position is quite magical, and invariably provokes the inquiry, how can
it be done? The study of numismatics, the science of coins and medals,
is generally considered to be limited to the taste of a very few
persons, and any description of a collection of coins at a lecture would
be voted a great bore, unless, of course, the members of the audience
happened to be antiquaries; great light, however, may be thrown on
history by a study of these interesting remains of bygone times, and a
lecture on this subject, illustrated with pictures of coins thrown on to
the disc by a concave mirror in the manner described, might be made very
pleasing and instructive.
Coins, or plaster casts of coins _gilt_, flowers, birds, white mice, the
human face and hands, may all, when fully illuminated, be reflected by
the concave mirror on to the disc. A Daguerreotype picture at a certain
angle appears, when reflected by the concave mirror, to be like any
ordinary collodion negative, and all the lights and shadows are
reversed, so that the face of the portrait appears black, whilst the
black coat is white. On placing the Daguerreotype in another position,
easily found by experiment, it is now reflected in the ordinary manner,
showing an enlarged and perfect portrait on the disc. In using the
Daguerreotype the glass in front of it must be removed. The pictures
from the concave mirror may be also projected on thick smoke procured
from [Page 287] smouldering damped brown paper, or from a mixture of
pitch and a little chlorate of potash laid on paper, and allowed to burn
slowly by wetting it with water.
An image reflected from smoke would be visible to a number of
spectators, just as the light from the furnace fires of the locomotive
is frequently visible at night, being reflected on the escaping column
of steam.
It was probably with the help of some kind of smoke and the concave
speculum that the deception practised on the worshippers at the temple
of Hercules at Tyre was carried out, as it is mentioned by Pliny that a
consecrated stone existed there "from which the gods easily rose." At
the temple of Esculapius at Tarsus, and that of Enguinum in Sicily, the
same kind of optical delusions were exhibited as a portion of the
religious ceremonies, from which no doubt the priests obtained a very
handsome revenue, much more than could be obtained in modern times by
the mere exhibition of such wonders at Adelaide Galleries, Polytechnics,
or Panopticons.
The smoke from brown paper is very useful in showing the various
directions of the rays of light when reflected from plane, convex, and
concave surfaces. The equal angles of the incident and reflected rays
may be perfectly shown by using the next arrangement of apparatus. (Fig.
276.)
[Illustration: Fig. 276. A. Rays of light slightly divergent issuing
from the lantern, and received on a little concave mirror, which brings
the rays almost parallel, and reflects them to E, a piece of
looking-glass, from which they are again reflected. C is the incident,
and D the reflected rays. F. Smoke from brown paper.]
A very dense white smoke is obtained by boiling in separate flasks (the
necks of which are brought close together) solutions of ammonia and
hydrochloric acid.
The opposite properties of convex and concave mirrors--the former
scattering and the latter collecting the rays of light which fall upon
them--are also effectively demonstrated by the help of the same
illuminating [Page 288] source and proper mirrors, the smoke tracing
out perfectly the direction of the rays of light. (Fig. 277.)
[Illustration: Fig. 277. The smoke shows the rays of light falling on a
convex mirror, and rendered still more divergent.]
The smoke developes the cone of rays reflected from a concave mirror in
the most beautiful manner, and by producing plenty of [Page 289] smoke,
and turning the mirror about--the position of the focus (_focus_, a
fire-place), is indicated by a brilliant spot of light, and the reason
the images of objects reflected by the concave mirror are reversed, may
be better understood by observing how the rays cross each other at that
point. (Fig. 278.)
[Illustration: Fig. 278. The smoke shows rays of light falling on the
concave mirror. In this experiment attention should be directed to the
bright point, E, the focus where the convergent rays meet.]
One of the most perfect applications of the reflection of light is shown
in the "Gregorian reflecting telescope," or in that magnificent
instrument constructed by Lord Rosse, at Parsonstown, in Ireland. (Fig.
279.)
[Illustration: Fig. 279. Lord Rosse's gigantic telescope.]
The description of nearly all elaborate optical instruments is somewhat
tedious, but we venture to give one diagram, with the explanation of the
Gregorian reflecting telescope. (Fig. 280.)
[Illustration: Fig. 280. The Gregorian reflecting telescope.]
At the bottom of the great tube T T T T, (Fig. 280), is placed the large
concave mirror D U V F, whose principal focus is at M; and in its middle
is a round hole P, opposite to which is placed the small mirror L,
concave towards the greater one, and so fixed to a strong wire M, that
it may be moved farther from the great mirror or nearer to it, by means
of a long screw on the outside of the tube, keeping its axis still in
the same line P M N with that of the great one. Now since in viewing a
very remote object we can scarcely see a point of it but what is at
least as broad as the great mirror, we may consider the rays of each
pencil, which flow from every point of the object, to be parallel to
each other, [Page 290] and to cover the whole reflecting surface D U V
F. But to avoid confusion in the figure, we shall only draw two rays of
a pencil flowing from each extremity of the object into the great tube,
and trace their progress through all their reflections and refractions
to the eye f, at the end of the small tube t t, which is joined to the
great one.
Let us then suppose the object A B to be at such a distance, that the
rays E flow from its lower extremity B, and the rays C from its upper
extremity A. Then the rays C falling parallel upon the great mirror at
D, will be thence reflected by converging in the direction D G; and by
crossing at i in the principal focus of the mirror, they will form the
upper extremity i of the inverted image i K, similar to the lower
extremity B of the object A B; and passing on the concave mirror L
(whose focus is at N) they will fall upon it at g and be thence
reflected, converging in the direction N, because g m is longer than g
n; and passing through the hole P in the large mirror, they would meet
somewhere about r, and form the lower extremity d of the erect image a
d, similar to the lower extremity B of the object A B. But by passing
through the plano-convex glass R in their way they form that extremity
of the image at b. In like manner the rays E which come from the top of
the object A B and fall parallel upon the great mirror at F, are thence
reflected converging to its focus, where they form the lower extremity K
of the inverted image i K, similar to the upper extremity A, of the
object A B; and passing on to the smaller mirror L and falling upon it
at h, they are thence reflected in the converging state h o; and going
on through the hole P of the great mirror, they would meet somewhere
about q, and form there the upper extremity a of the erect image a d,
similar to the upper extremity A of the object A B; but by passing
through the convex glass R, in their way, they meet and cross sooner, as
at a, where that point of the erect image is formed. The like being
understood of all those rays which flow from the intermediate points of
the object, between A and B, and enter the tube T T, all the
intermediate points of the image between a and b will be formed; and the
rays passing on from the image through the eye-glass S, and through a
small hole e in the end of the lesser tube t t, they enter the eye f
which sees the [Page 291] image a d (by means of the eye-glass), under
the large angle c e d, and magnified in length, under that angle, from c
to d.
To find the magnifying power of this telescope, multiply the focal
distance of the great mirror by the distance of the small mirror, from
the image next the eye, and multiply the focal distance of the small
mirror by the focal distance of the eye-glass; then divide the product
of the latter, and the quotient will express the magnifying power. (Fig.
280.)
We now come to that much disputed and often quoted experiment of
Archimedes, who is stated to have employed metallic concave specula or
some other reflecting surface by which he was enabled to set fire to the
Roman fleet anchored in the harbour of Syracuse, and at that time
besieging their city, in which the great and learned philosopher was
shut up with the other inhabitants. The story handed down to posterity
was not disputed till about the seventeenth century, when Descartes
boldly attacked the truth of it on philosophical grounds, and for the
time silenced those who supported the veracity of this ancient Joe
Miller. Nearly a hundred years after this time, the neglected Archimedes
fiction was again examined by the celebrated naturalist Buffon, and the
account of his experiments detailed by the author of "Adversaria," in
Chambers' Journal, is so logical and conclusive, that we give a portion
of it verbatim.
"For some years prior to 1747, the French naturalist Buffon had been
engaged in the prosecution of those researches upon heat which he
afterwards published in the first volume of the Supplement to his
'Natural History.' Without any previous knowledge, as it would seem, of
the mathematical treatise of Anthemius ([Greek: _peri paradoxôn
mêchanêmatôn_]), in which a similar invention of the sixth century is
described,[G] Buffon was led, in spite of the reasonings of Descartes,
to conclude that a speculum or series of specula might be constructed
sufficient to obtain results little, if at all, inferior to those
attributed to the invention of Archimedes.
[Footnote G: See Gibbon's "Decline and Fall," chap. xl., section v.,
note _g_.]
"This, after encountering many difficulties, which he had foreseen with
great acuteness, and obviated with equal ingenuity, he at length
succeeded in effecting. In the spring of 1747, he laid before the French
Academy a memoir which, in his collected works, extends over upwards of
eighty pages. In this paper, he describes himself as in possession of an
apparatus by means of which he could set fire to planks at the distance
of 200, and even 210 feet, and melt metals and metallic minerals at
distances varying from twenty-five to forty feet. This apparatus he
describes as composed of 168 plain glasses, silvered on the back, each
six inches broad by eight inches long. These, he says, were ranged in a
large wooden frame, at intervals not exceeding the third of an inch; so
that, by means of an adjustment behind, each should be moveable in all
directions independently of the rest--the spaces between the glasses
being further of use in allowing the operator to see from behind the
point on which it behoved the various disks to be converged.
[Page 292]
"These results ascertained, Buffon's next inquiry was how far they
corresponded with those ascribed to the mirrors of Archimedes--the most
particular account of which is given by the historians Zonaras and
Tzetzes, both of the twelfth century.[H] 'Archimedes,' says the first of
these writers, 'having received the rays of the sun on a mirror, by the
thickness and polish of which they were reflected and united, kindled a
flame in the air, and darted it with full violence on the ships which
were anchored within a certain distance, and which were accordingly
reduced to ashes.' The same Zonaras relates that Proclus, a celebrated
mathematician of the sixth century, at the siege of Constantinople, set
on fire the Thracian fleet by means of brass mirrors. Tzetzes is yet
more particular. He tells us, that when the Roman galleys were within a
bow-shot of the city-walls, Archimedes caused a kind of hexagonal
speculum, with other smaller ones of twenty-four facets each, to be
placed at a proper distance; that he moved these by means of hinges and
plates of metal; that the hexagon was bisected by 'the meridian of
summer and winter;' that it was placed opposite the sun; and that a
great fire was thus kindled, which consumed the Roman fleet.
[Footnote H: Quoted by Fabricius in his "Biblioth. Græc.," vol. ii., pp.
551, 552.]
"From these accounts, we may conclude that the mirrors of Archimedes and
Buffon were not very different either in their construction or effects.
No question, therefore, could remain of the latter having revived one of
the most beautiful inventions of former times, were there not one
circumstance which still renders the antiquity of it doubtful: the
writers contemporary with Archimedes, or nearest his time, make no
mention of these mirrors. Livy, who is so fond of the marvellous, and
Polybius, whose accuracy so great an invention could scarcely have
escaped, are altogether silent on the subject. Plutarch, who has
collected so many particulars relative to Archimedes, speaks no more of
it than the former two; and Galen, who lived in the second century, is
the first writer by whom we find it mentioned. It is, however, difficult
to conceive how the notion of such mirrors having ever existed could
have occurred, if they never had been actually employed. The idea is
greatly above the reach of those minds which are usually occupied in
inventing falsehoods; and if the mirrors of Archimedes are a fiction, it
must be granted that they are the fiction of a philosopher."
Supposing that Archimedes really did project the concentrated rays of
the sun on the Roman vessels, one cannot help pitying the ignorance of
the Admiral Marcellus. Had this officer been acquainted with the laws of
the reflection of light, he might have laughed to scorn the power of
Archimedes, and by receiving the unfriendly rays on one of the bright
brazen convex shields of his soldiers, Marcellus could have scattered
the concentrated rays, and prevented the burning of his vessels.
In these days of learning it therefore appears strange to find any one
advocating the possible use of specula or reflecting mirrors for the
purposes of offence or defence, but M. Peyrard a few years ago proposed
[Page 293] to produce great effects by mounting each mirror in a
distinct frame, carrying a telescope so that one person could direct the
rays to the object intended to be set on fire, and he gravely
calculated, presuming on the ignorance of the attacked, that with 590
glasses of about twenty inches in diameter, he could reduce a fleet to
ashes at the distance of a quarter of a league! and with glasses of
double that size at the distance of half a mile! What effect a shell or
shot would produce upon this ancient weapon is not stated; this we may
safely leave our readers to determine for themselves. The experiment of
Archimedes has long been a favourite one with the boys. (Fig. 281.)
[Illustration: Fig. 281. One of the "miseries of _reflection_."]
The total internal reflection of light by a column of water is an
experiment that admits of great variety so far as colour is concerned,
and is one of the most novel and beautiful experiments with light
presented to the public within the last few years. The author had the
pleasure of introducing it in the first place at the Polytechnic
Institution, where the optical novelty excited the greatest attention,
and received the approbation of her Most Gracious Majesty, and his Royal
Highness the Prince Consort, with the Royal Family, who were pleased to
pay a private evening visit to the Polytechnic, and amongst other things
minutely examined the "Illuminated Cascade," which had been erected by
Mons. Duboscq of Paris.
The illumination of the descending columns of water was obtained by
converging the rays from a powerful electric light upon the orifice
from [Page 294] which the water escaped, the Duboscq lantern already
explained being employed, and in front of it were placed three
cylinders, each having a circular window behind and opposite the lens,
and an aperture of about one inch in diameter on the opposite side for
the escape of water. The lantern used was of a peculiar shape, and had
three sides, the electric light being in the centre of them, and passing
through three separate plano-convex lenses to the three cylinders from
which the water escaped.
[Illustration: Fig. 282.--Fig. 1. A. The electric light. B C D. The
three sides and lenses of the lantern. E F G. The three cylinders of
water, each with a circular glass window and orifices at Z Z Z, from
which the water and rays of light pass out.--Fig. 2. H. Section of one
side of the Duboscq lantern. I I. Cylinder of water, which enters from
below. K K. The stream of illuminated water. L L. Bit of coloured glass
held between the lantern and the cistern of water.]
Attention may be directed to the fact that the light merely passes out
of the orifices as a diverging beam of light until the flow of water
commences, when the rays are immediately taken up and reflected from
[Page 295] point to point inside the arched column of water, and
illuminating the latter in the most lovely manner, it appears sometimes
like a stream of liquid metal from the iron furnace, or like liquid ruby
glass, or of an amethyst or topaz colour, according to the colours of
the plates of glass held between the mouths of the lantern and the
circular windows in the cylinders of water. The same experiment created
quite a _furore_ at the Crystal Palace when it was introduced in one of
the author's lectures delivered in that noble place of amusement. In
order that our readers may understand the arrangement of the apparatus,
we have given at page 294 a ground plan view of it, as also the
appearance of the cascade when exhibited at the Polytechnic to the Royal
party. (Fig. 284.)
[Illustration: Fig. 283. A B. The sides of the cascade. The dotted lines
show the reflection of only two rays of the beam of light passing down
inside the water.]
Another curious effect observed with the illuminated cascade, is the
descent of balls of light as the reflection is cut off for a moment by
passing the finger through the stream of water, showing that a certain
time is occupied in the reflection of light from one end of the cylinder
of water to the other; indeed the best idea of the _rationale_ of the
experiment is formed by substituting in imagination a silver tube highly
polished in the interior, for the descending jet of water. The
reflection of sound takes place precisely in the same manner, and the
vibrations of the air are reflected from plane, concave, and convex
surfaces. It is on this principle that waves of sound thrown off from
different surfaces (as of hard rocks), produce the effect of the _echo_.
The sounds arrive at the ear in succession, those reflected nearest the
ear being first, and the reflecting surfaces at the greatest distance
sending the waves of sound to the ear after the former. At Lurley Falls
on the Rhine, there is an echo which repeats seventeen times. Whispering
galleries, again, illustrate the reflection of sound from continuous
curved surfaces, just as the arched column of water reflects from its
interior curved surfaces the rays of light.
Speaking-tubes are well known in which the waves of sound are
successively reflected from the sides, exactly like the "Illuminated
Cascade" (Fig. 283). The speaking-trumpet is also another and familiar
example of the same principle. Probably when Albertus Magnus constructed
the brazen head, which had the power of talking, it was nothing more
than a metallic head with a few wheels and _visible_ mechanism inside,
but connected with a lower apartment by a hollow metal tube, where
Albertus Magnus descended, and astonished the ignorant with [Page 296]
the then unknown principle of the speaking tube. Light entering at one
end of a bright metallic tube is reflected from the sides of the tube
till it reaches the other, and precisely the same effect occurs in the
interior of the cascade of water. (Fig. 284).
[Illustration: Fig. 284. End of Polytechnic Hall, where the illuminated
cascade was displayed to her Majesty, H.R.H. the Prince Consort, and
Royal party. The cascades issued from behind some artificial rock-work.]
THE KALEIDOSCOPE.
If this article on light and optics had gone minutely into the
mathematical and purely scientific portion of the subject, we should
have had frequent occasion to mention the name of Sir David Brewster, a
distinguished philosopher, whose name is peculiarly identified with this
interesting branch of physics. It is always pleasing to find men of such
standing not only devoting themselves to arguments which college
wranglers would study with pleasure, but also descending to a lower
level, and inventing optical instruments that delight and amuse the
non-scientific and juvenile part of the community. The names of Sir
David Brewster and Professor Wheatstone have been connected during the
last few years with the invention of the stereoscope, an instrument
[Page 298] that will be noticed in another part of this book, but here
we shall describe one of the most original optical instruments ever
devised, and although it is now regarded as a mere toy, its merits are
very great. The title of the instrument is borrowed from the Greek
[Greek: _kalos_], beautiful, [Greek: _eidos_], a form or appearance,
[Greek: _skopeô_], to see; and the public certainly endorsed the name
when they purchased 200,000 of these instruments in London and Paris
during the space of three months. It is said that the sensation it
excited in London, throughout all ranks of the community, was
astonishing, and people were everywhere seen, even at the corners of the
streets, looking through the kaleidoscope. The essential parts of this
instrument are two mirrors of unsilvered black parallel glass, or plate
glass painted black on one side, which should be from six to ten inches
in length, and from one inch to an inch and a half in breadth at the
object end, while they are made narrower at the other end, to which the
eye is applied. The mirrors are united at their lower edges by a strip
of black calico fixed with common glue, and are left open at the upper
edges, and retained at the proper angle by a bit of cork properly
blackened. The angles are 36°, 30°, 25°-5/7, 22½°, 20°, 18°, which
divide the circumference into 10, 12, 14, 16, 18, 20 parts, thus 36 × 10
= 360, or 18 × 20 = 360, and the strictest attention must be paid to
this part of the adjustment, or the figures produced will not be
symmetrical. After the mirrors are adjusted to [Page 298] the proper
angle, the space between the two upper edges should be covered across
with black velvet and the mirrors placed in a tin or brass tube, so that
the broad ends shall barely project beyond the end, while the narrow end
is placed so that the angle formed by the junction of the mirrors shall
be a little below the middle of that end of the tube. A cover with a
circular aperture in the centre is then to be fitted to the narrow end
of the mirrors, which should in general be furnished with a convex lens
whose focal length is an inch or two greater than the length of the
mirrors. A case for holding the objects, and for communicating to them a
revolving motion, is fitted to the object end of the tube. The objects
best suited for producing pleasing effects are small fragments of
coloured glass, wires of glass, both spun and twisted, and of different
colours and shades of colours, and of various shapes, in curves, angles,
circles; also, beads, bugles, fine needles, small pieces of lace, and
fragments of fine sea-weed are very beautiful. M. Sturm, of Prague, has
lately fixed the images of the kaleidoscope, so that they are available
for the production of patterns in every branch of silk, cotton, and
mixed fabrics. Photographs could be taken of the most beautiful of these
accidental designs, which only occur once, and if not copied are lost.
[Illustration: Fig. 285. A B. The tube containing the two mirrors, shown
by dotted lines. A. is the small end where the eye is placed. B. The
object end. C D. Another view of the mirrors arranged to place in the
tube; the shaded portion represents the black velvet. E. Double convex
lens. F. Box to contain objects, and usually fitted with ground glass
outside.]