phenomena of polarized light. They all consist essentially of a
polarizer and an analyser, which may be two mirrors of black glass
placed at the polarizing angle, or two bundles of thin glass plates, or
two Nicol’s prisms, or two plates of tourmaline, or any pair formed by
two of these. Fig. 209 represents a polariscope, this instrument being
designed to permit any desired combination of polarizer and analyser,
and having graduations for measuring the angles, and a stage upon which
may be placed various substances in order to observe the effects of
polarized light when transmitted through them. It is found that thin
slices of crystals placed between the polarizer and analyser exhibit
varied and beautiful effects of colour, and by such effects the doubly
refracting power of substances can be recognized, where the observation
of the production of double images would, on account of their small
separation, be impossible. And the polariscope is of great service in
revealing structures in bodies which with ordinary light appear entirely
devoid of it—such, for example, as quill, horn, whalebone, &c. Except
liquids, well-annealed glass, and gelatinous substances, there are, in
fact, few bodies in which polarized light does not show us the existence
of some kind of structure. A very interesting experiment can be made by
placing in the apparatus, shown in Fig. 210, a square bar of
well-annealed glass; on examining it by polarized light, it will be
found that before any pressure from the screw C is applied to the glass,
it allows the light to pass equally through every part of it; but when
by turning the screw the particles have been thrown into a state of
strain, as shown in the figure, distinct bands will make their
appearance, arranged somewhat in the manner represented; but the shapes
of the figures thus produced vary with every change in the strain and in
the mode of applying the pressure.
[Illustration:
FIG. 212.
]
_CAUSE OF LIGHT AND COLOUR._
We have hitherto limited ourselves to a description of some of the
phenomena of light, without entering into any explanation of their
presumed causes, or without making any statements concerning the nature
of the agent which produces the phenomena. Whatever this cause or agent
may be, we know already that light requires time for its propagation,
and two principal theories have been proposed to explain and connect the
facts. The first supposes light to consist of very subtile matter shot
off from luminous bodies with the observed velocity of light; and the
second theory, which has received its great development during the
present century, regards luminous effects as being due to movements of
the particles of a subtile fluid to which the name of “ether” has been
given. Of the existence of this ether there is no proof: it is imagined;
and properties are assigned to it for no other reason than that if it
did exist and possess these properties, most of the phenomena of light
could be easily explained. This theory requires us to suppose that a
subtile imponderable fluid pervades all space, and even interpenetrates
bodies—gaseous, liquid, and solid; that this fluid is enormously
elastic, for that it resists compression with a force almost beyond
calculation. The particles of luminous bodies, themselves in rapid
vibratory motion, are supposed to communicate movement to the particles
of the ether, which are displaced from a position of equilibrium, to
which they return, executing backwards and forwards movements, like the
stalks of corn in a field over which a gust of wind passes. While an
ethereal particle is performing a complete oscillation, a series of
others, to which it has communicated its motion, are also performing
oscillations in various phases—the adjacent particle being a little
behind the first, the next a little behind the second, and so on, until,
in the file of particles, we come to one which is in the same phase of
its oscillation as the first one. The distance of this from the first is
called the “length of the luminous wave.” But the ether particles do
not, like the ears of corn, sway backwards and forwards merely in the
direction in which the wave itself advances: they perform their
movements in a direction perpendicular to that in which the wave moves.
This kind of movement may be exemplified by the undulation into which a
long cord laid on the ground may be thrown when one end is violently
jerked up and down, when a wave will be seen to travel along the cord,
but each part of the latter only moves perpendicularly to the length.
The same kind of undulation is produced on the surface of water when a
stone is thrown into a quiet pool. In each of these cases the parts of
the rope or of the water do not travel along with the wave, but each
particle oscillates up and down. Now, it may sometimes be observed, when
the waves are spreading out on the surface of a pool from the point
where a stone has been dropped in, that another set of waves of equal
height originating at another point may so meet the first set, that the
crests of one set correspond with the hollows of the other, and thus
strips of nearly smooth water are produced by the superposition of the
two sets of waves. Let Fig. 212 represent two systems of such waves
propagated from the two points A A, the lines representing the crests of
the waves. Along the lines, _b b_, the crests of one set of waves are
just over the hollows of the other set; so that along these lines the
surface would be smooth, while along C C the crests would have double
the height. Now, if light be due to undulation, it should be possible to
obtain a similar effect—that is, to make two sets of luminous
undulations destroy each other’s effects and produce darkness: in other
words, we should be able, _by adding light to light, to produce
darkness!_ Now, this is precisely what is done in a celebrated
experiment devised by Fresnel, which not only proves that darkness may
be produced by the meeting of rays of light, but actually enables us to
measure the lengths of the undulations which produce the rays.
[Illustration:
FIG. 213.
]
In Fig. 213 is a diagram representing the experiment of the two mirrors,
devised by Fresnel. We are supposed to be looking down upon the
arrangement: the two plane mirrors, which are placed vertically, being
seen edgeways, in the lines, M O, O N, and it will be observed that the
mirrors are placed _nearly_ in the same upright plane, or, in other
words, they form an angle with each other, which is nearly 180°. At L is
a very narrow upright slit, formed by metallic straight-edges, placed
very close together, and allowing a direct beam of sunlight to pass into
the apartment, this being the only light which is permitted to enter.
From what has been already said on reflection from plane mirrors, it
will readily be understood that these mirrors will reflect the beams
from the slit in such a manner as to produce the same effect, in every
way, as if there were a real slit placed behind each mirror in the
symmetrical positions, A and B. Each virtual image of the slit may,
therefore, be regarded as a real source of light at A and at B; thus,
for example, it will be observed that the actual lengths of the paths
traversed by the beams which enter at L, and are reflected from the
mirrors, are precisely the same as if they came from A and B
respectively. The virtual images may be made to approach as near to each
other as may be required, by increasing the angle between the two
mirrors, for, when this becomes 180°, that is, when the two mirrors are
in one plane, the two images will coincide. If, now, a screen be placed
as at F G, a very remarkable effect will be seen; for, instead of simply
the images of the two slits, there will be visible a number of vertical
coloured bands, like portions of very narrow rainbows, and these
coloured bands are due to the two sources of light, A and B; for, if we
cover or remove one of the mirrors, the bands will disappear and the
simple image of the slit will be seen. If, however, we place in front of
L a piece of coloured glass, say red, we shall no longer see
rainbow-like bands on the screen, but in their place we shall find a
number of strips of red light and dark spaces alternately, and, as
before, these are found to depend upon the _two_ luminous sources, A and
B. We must, therefore, come to the conclusion that the two rays exercise
a mutual effect, and that, by their superposition, they produce darkness
at some points and light at others. These alternate dark and light bands
are formed on the screen at all distances, and the spaces between them
are greater as the two images, A and B, are nearer together. Further,
with the same disposition of the apparatus, it is found that when yellow
light is used instead of red, the bands are closer together; when green
glass is substituted for yellow, blue for green, and violet for blue,
that the bands become closer and closer with each colour successively.
Hence, the effect of coloured bands, which is produced when pure
sunlight is allowed to enter at L, is due to the superposition of the
various coloured rays from the white light. Let us return to the case of
the red glass, and suppose that the distance apart of the two images, A
and B, has been measured, by observing the angle which they subtend at
C, and by measuring the distance, C O D, or rather, the distance C O L.
Now, the distances of A and B from the centre of each dark band, and of
each light band, can easily be calculated, and it is found that the
_difference between the two distances_ is always the same for the same
band, however the screen or the mirrors may be changed. On comparing the
_differences_ of the distances of A and B in case of bright bands, with
those in the case of dark ones, it was found that the former could be
expressed by the even multiples of a very small distance, which we will
call _d_, thus:
0, 2_d_, 4_d_, 6_d_, 8_d_, ...
while the differences for the dark bands followed the odd multiples of
the same quantity, _d_, thus:
_d_, 3_d_, 5_d_, 7_d_, 9_d_, ....
These results are perfectly explained on the supposition that light is a
kind of wave motion, and that the distance, _d_, corresponds to _half
the length of a wave_. We have the waves entering L, and pursuing
different lengths of path to reach the screen at F G, and, if they
arrive in opposite phases of undulation, the superposition of two will
produce darkness. The undulations will plainly be in opposite phases
when the lengths of paths differ by an _odd_ number of _half-wave_
lengths, but in the same phase when they differ by an _even_ number.
Hence, the length of the wave may be deduced from the measurement of the
distances of A and B from each dark and light band, and it is found to
differ with the colour of the light. It is also plain that, as we know
the velocity of light, and also the length of the waves, we have only to
divide the length that light passes over in one second, by the lengths
of the waves, in order to find how many undulations must take place in
one second. The following table gives the wave-lengths, and the number
of undulations for each colour:
┌───────┬──────┬───────────────────┐
│Colour.│Number│ Number of │
│ │ of │Oscillations in one│
│ │Waves │ second. │
│ │in one│ │
│ │inch. │ │
├───────┼──────┼───────────────────┤
│Red │40,960│514,000,000,000,000│
│Orange │43,560│557,000,000,000,000│
│Yellow │46,090│578,000,000,000,000│
│Green │49,600│621,000,000,000,000│
│Blue │53,470│670,000,000,000,000│
│Indigo │56,560│709,000,000,000,000│
│Violet │60,040│750,000,000,000,000│
└───────┴──────┴───────────────────┘
These are the results, then, of such experiments as that of Fresnel’s,
and although such numbers as those given in the table above are apt to
be considered as representing rather the exercise of scientific
imagination than as real magnitudes actually measured, yet the reader
need only go carefully over the account of the experiment, and over that
of the measurement of the velocity of light, to become convinced that by
these experiments _something_ concerned in the phenomena of light has
really been measured, and has the dimensions assigned to it, even if it
be not actually the distance from crest to crest of ether waves—even,
indeed, if the ether and its waves have no existence. But by picturing
to ourselves light as produced by the swaying backwards and forwards of
particles of ether, we are better able to think upon the subject, and we
can represent to ourselves the whole of the phenomena by a few simple
and comparatively familiar conceptions.
As an example of the facility with which the ether theory lends itself
to aiding our notions of the phenomena of light, take the explanation of
polarization. Let us suppose that we are looking at a ray of light along
its direction, and that we can see the particles of ether. We should, in
such a case, see them vibrating in planes having every direction, and
their paths, as so seen, would be represented by an indefinite number of
the diameters of a circle. Now, suppose we make the ray first pass
through a rhomb of Iceland spar: we should, if we could see the
vibrating particles in the emergent ordinary and extraordinary rays,
perceive them swaying backwards and forwards across the direction of the
rays in two planes only, as represented by the lines, B D and A C, in
the two circles, O _o_ and E _e_, Fig. 214–-that is, half the particles
would be vibrating in the direction B D, and the other half in the
direction A C; and further, the two directions would be at right angles
to each other—the vibrations forming the extraordinary ray being
performed in a plane at right angles to that in which the vibrations
producing the ordinary ray take place. If—these planes being in the
position indicated in 1, Fig. 214–-we turn the crystal round through
90°, they would rotate with it, and would come severally into the
position shown in 2, Fig. 214.
[Illustration:
FIG. 214.
]
It was at one time objected to the theory which represents light as due
to wave-like movements that, just as the vibrations which constitute
sound spread in all directions, and go round intercepting bodies,
enabling us, for example, to hear the sound of a bell even when a
building intervenes, so if vibrations really produce light, these would
extend within the shadows, and we ought to perceive light within the
shadows, bending, as it were, round the edge of the shadow-casting body.
This objection, which at one time presented a great difficulty for the
wave theory, was triumphantly removed by the discovery that the luminous
vibrations do extend into the shadow, and that this is in reality never
completely dark. It is true that, although we can hear round a corner,
we are in general unable to see round it; but it should be noticed that
in the case of hearing, the sound is much weakened by intervening
objects, and that there are what may be termed _sound shadows_. A ray of
light produces sensible effects only in the direction of its
propagation; but it can be shown that the successive portions of the
waves advancing along it are centres of lateral disturbances producing
new or secondary waves in all directions, which, however, interfere with
and destroy each other. When an opaque screen intercepts a portion of
the principal wave, it also stops a number of oblique or secondary
waves, which would interfere more or less with the rest. Under ordinary
circumstances, the remaining oblique or secondary rays are quite
insensible in the presence of the direct light. But, with an apparatus
which will cost but the two or three minutes’ time required to construct
it, the reader may see for himself that light is able to _pass round an
obstacle_, and he may witness directly phenomena of the same order as
those presented in the experiment of Fresnel’s mirrors, which require
costly apparatus for their production. He has only to take two fragments
of common window-glass, and having made a piece of tinfoil adhere to one
surface of each piece of glass, cut, with a sharp penknife, the finest
possible slit in each piece of tinfoil, making the slit from ½ in. to 1
in. in length. If he will then hold one piece of glass about 2 ft. from
his eye, so that it may be in the line between his eye and the sun (or
other luminous body), and hold the other piece close to his eye with its
slit parallel to that in the first piece, he will see the latter not
simply as a line of light, but parallel to it a number of
brilliantly-coloured rainbow-like bands will be seen on either side. If,
instead of receiving the light from the sun, or from a candle-flame, the
light given off by a spirit-lamp, with a piece of salt on its wick, be
used, bright yellow stripes will be seen with dark spaces between them.
Or, if the piece of glass next the sun be red-coloured, instead of plain
glass, no rainbow-like bands will be visible, but a number of bright red
stripes alternating with dark bands will be seen. The reader will have
probably now little difficulty in perceiving that these can be easily
explained as the results of interferences of a kind quite analogous to
those of the waves of water represented in the diagram, Fig. 212. The
rainbow-like stripes are due to the different wave-lengths of the
different colours, as a consequence of which the bright and dark bands
would be formed at different positions. Our limits do not admit of a
full explanation of these beautiful effects, but the reader requiring
further information would peruse with the greatest advantage portions of
Sir John Herschel’s “Familiar Lectures on Scientific Subjects.”
The undulatory theory gives also an easy explanation of colours; they
being, according to the theory, only the effects, as already stated, of
the different rates of vibrations of the ether. If the ether particles
perform 514,000,000,000,000 oscillations in a second, we receive the
impression we call red colour; if they execute 750,000,000,000,000
vibrations, the impression produced on our organ of sight is
different—we call it violet; and so on. Thus science teaches us that
visual impressions so different as red, green, blue, violet, and other
distinct colours, are, in reality, all due to movements of one and the
same——something; and that the different sensations of colour we
experience, arise merely from different rates of recurrence in these
movements. In the subsequent article we shall have occasion to show that
ordinary light, such as that of the sun, or of a candle, contains rays
of every imaginable colour, mixed together in such proportions, that
when this light falls upon a piece of paper, or upon snow, we have, in
looking at these objects, the sensation of _whiteness_. But, if the
light falls upon any substance which is able, in some way, to absorb or
destroy some of the vibrations, the admixture of which makes up “_white
light_,” as it is called, then that object sending back to our eyes the
rays formed of the remaining group of vibrations, gives us the sensation
of _colour_. Suppose, for example, a substance to be so constituted that
it is capable of absorbing, or quenching in some way, all the vibrations
of the ether which occur at a quicker rate than 520,000,000,000,000 in a
second: such a substance would send back to our eyes only the vibrations
which constitute red light (see table, page 411), and we should say the
substance in question had a _red_ colour. Similarly, if the substance
gave back only the vibrations which have the quickest rates, we should
call the substance of a _violet_ character. The agent which produces in
our visual organs the impression of colour is, therefore, not in the
objects, but in the light which falls upon them. The rose is red, not
because it has redness in itself, but because the light which falls upon
it contains some rays in which there are movements that occur just the
number of times per second that gives us the impression we call redness;
in short, the colour comes not from the flower but from the light.
“But,” the reader might say, “the rose is always red by whatever light I
see it, and therefore the colour must be in the flower. Whether I view
it by sunlight, or moonlight, or candlelight, or gaslight, I invariably
see that _it is red_.” Now, it is precisely this circumstance—the
seemingly invariable association of the object with a certain
impression—in this case, redness—that leads our judgment astray, and
makes us believe that the colour is in the object. Most people live out
their lives without anything occurring to them which would give them the
least idea that the colours of the objects they see around them are not
in these objects themselves, but are derived from the light that falls
upon the objects. And it required the comparison of many observations
and experiments, and some clear reasoning, to establish a truth so
unlike the most settled convictions of ordinary minds.
The point in question is fortunately one extremely easy of experiment,
since we have simple means of producing light in which the vibrations
corresponding to only one colour are present. The reader is strongly
recommended to try the following experiment for himself. Let him procure
a spirit-lamp, and place on the wick a piece of common salt about as
large as a pea. Let the lamp be lighted in a room from which all other
light is completely excluded, and bring near the flame a red rose or a
scarlet geranium. The flower will be seen with _all its redness gone_—it
will appear of an ashy grey or leaden colour. A ball of bright scarlet
wool, such as ladies use to work brilliant patterns for cushions, &c.,
held near this flame, is apparently transformed into a ball of the
homely grey worsted with which, about a century ago, old ladies might be
seen industriously darning stockings. The experiment is, perhaps, even
more striking when, a little distance from the spirit-lamp, is placed a
feeble light of the ordinary kind, a rushlight for example. The ball of
wool, held near the latter, shows vivid scarlet, but, brought near the
spirit-lamp with the salted wick, is pale, ashy grey. Moving thus the
ball of worsted, first to one light then to the other, gives a most
convincing and striking proof of the entire illusion we are under as to
colour being an inherent quality of substances. Similar experiments may
be multiplied indefinitely. A bouquet, viewed by the rushlight, shows
the so-called _natural_ colours of the flowers; viewed by the salted
flame, roses, verbenas, violets, larkspurs, and leaves, all appear of
the uniform ashy grey, and only _yellow_ flowers come out in their
_natural_ colours. A picture, say a chromo-lithograph after one of the
most gorgeous landscapes that Turner ever painted, appears a work in
monochrome, and gives exactly the effect of a sepia or indian-ink
drawing. The most blooming complexion vanishes, and the countenance
assumes a cadaverous aspect very startling to persons of weak nerves;
the lips especially, which might have rivalled pink coral by ordinary
light, take a repulsive livid hue. All these effects may be seen to
greater advantage by using the gas-flame of a Bunsen’s burner, having a
lump of salt placed in the flame; or by means of a piece of _fine_ wire
gauze, about six inches square, supported about two or three inches
above an ordinary gas-burner, from which the gas is allowed to issue
without being lighted, but when to the top of the wire gauze, which is
strewed with small fragments of salt, a light is applied, the gas will
ignite only above the gauze, without the flame passing down to the
burner below.
A fuller explanation of these strange appearances may be gathered from
the subsequent article; but it may suffice now to state that spirit, or
gas burned in the way we have indicated, gives off little or no light of
any kind. If, however, common salt be introduced into the flame, then
light—but light of only one particular colour—is given off, and that
colour is yellow. There are no red, or green, or blue, or violet
vibrations given off; and as the objects on which the light falls cannot
supply these, it follows that with this light no impression
corresponding to these colours can be produced on the eye, whatever may
be the objects upon which it falls. Such experiments, not simply read
about but actually performed, cannot fail to convince an intelligent
person that the colours come from the light and not from the object. Of
course, it is not denied that there is in each substance something that
determines which are the rays absorbed, and which are the rays reflected
to the eye—something that can destroy certain waves, but is powerless
over others that rebound from the substance, and reaching the eye, there
produce their characteristic impressions. And it is but this power of
sending back only certain rays among the multitude which a sunbeam
furnishes, that can be attributed to objects when we say that they have
such or such a colour. In this sense, then, we may properly say that
_the rose is red_, but it is also at the same time undeniably true that
_the redness is not in the rose_.