LIGHT, A MODE OF MOTION
ART. 70. _Light, a Mode of Motion._--No subject has in the past received
greater attention from philosophers and scientists than that involved in
the question as to "What is Light?" Indeed, it may truthfully be said,
that even to-day its exact character is not positively known. That it is
due like heat to some periodic wave motion in the Aether is known, but
the exact character of that wave motion has yet to be determined. As in
the case of heat, so in the case of light, there have been two theories
which have contended with each other for supremacy in endeavouring to
answer the question as to "What is Light?" Those two theories are known
as the Emission or Corpuscular Theory, and the Undulatory or Wave
Theory. The corpuscular theory was introduced and developed by Newton in
his work on _Optics_, which ranks second only to the _Principia_ as a
work revealing masterly research and scientific genius. Newton supposed
that a luminous or lighted body actually emitted minute particles, which
were shot out from the body with the velocity of light, that is, at the
rate of 186,000 miles per second. These minute particles he termed
corpuscles. In the work just referred to regarding this matter, he asks
the question, "Are not rays of light very small bodies emitted from
shining substances?" These small particles or corpuscles were supposed
by him to actually strike the retina of the eye, and so produce the
sensation of Sight, in the same way that odorous particles entering the
nostril, come into contact with the olfactory nerves and produce the
sensation of Smell. In order, however, to account for certain phenomena
of light, he was compelled to postulate an aetherial medium to fill all
space, in which his luminous corpuscles travelled, and which would
excite waves in that medium. In his eighteenth query on this point he
asks: "Is not the heat of a warm room conveyed through the vacuum by the
vibration of a much subtler medium than air, and is not this medium the
same with that medium by which light is reflected or refracted, and by
whose vibrations light communicates Heat to bodies, and is put into fits
of easy reflection and easy transmission?" The corpuscular theory,
however, received its death-blow when, in competition with the wave
theory of light, as developed by Young, it was found that the latter
theory satisfactorily accounted for certain phenomena as the refraction
of light, which the corpuscular theory did not adequately account for.
Even while Newton was developing his theory, Huyghens, a contemporary of
Newton, was developing another theory which is now known as the
undulatory or wave theory. Huyghens drew his conclusions from the
analogy of sound. He knew that sounds were propagated by waves through
the air, and from the region of the known, endeavoured to carry the
principle into the region of the unknown, a strictly philosophical
method, and one in accordance with the second Rule of Philosophy. He
supposed that light, therefore, like sound, might be due to wave motion,
but if it were wave motion, there must have been a medium to propagate
the waves. In order to account for this wave motion, he supposed all
space to be filled with a luminiferous Aether, which would be to his
light waves what air is to sound waves. In this conception he was
supported by Euler the mathematician, and in 1690 he was able to give a
satisfactory explanation of the reflection and refraction of light, on
the hypothesis that light was due to wave motion in the Aether. It was
not, however, till the advent of Thomas Young, that the undulatory or
wave theory reached its perfection, and finally overthrew its competitor
the corpuscular theory. Young made himself thoroughly acquainted with
wave motion of all kinds, and applied his knowledge and experience to
the phenomena of light, and from the analogies so obtained, he gradually
built up the undulatory theory, and gave to it a foundation from which
it has not yet been moved. Young made use of the same aetherial medium
in order to propagate the wave motion of light in the same way that
Huyghens did. From that conception, the Aether has been gradually
perfected, until we have the conception which has been presented to the
reader in Chapter IV., in which I have endeavoured to show that this
aetherial medium is matter, but infinitely more rarefied and infinitely
more elastic, but notwithstanding its extreme rarefaction and
elasticity, it possesses inertia, because it is gravitative. It is this
Aether, then, that is concerned in the propagation of light, and is the
universal medium which is to light what air is to sound. Young,
therefore, having applied himself to the wave motion of sound, from such
researches was able to explain the physical cause of colour, and that
phenomenon termed interference.
We will therefore look at wave motion, in order to understand the wave
theory of light.
Now in all wave motion, whether it be water waves or sound waves, that
which is propagated or conveyed from place to place is energy, or
motion. If a stone is thrown into water, a series of concentric circles
of waves are generated, which spread out with increasing size, but
decreasing power or motion, regularly on all sides. The water, however,
does not move away from the generating source. There is a motion of the
water, but it is simply a wave motion, so that the propagation of a wave
is the propagation of motion, rather than the transference of the actual
water which constitutes the wave. In the case of sound waves, we have
again an illustration of the same principle. For example, suppose we
strike a bell, and so set the particles of that bell in a state of
vibration. These vibrations give the air in contact with the bell a
forward movement, and then, owing to the elasticity and inertia of the
air, a backward movement is set up, with the result that a series of
waves are set in motion from the bell on every side, which gradually
diminish in intensity the farther they recede from the generating body.
According to the wave theory, therefore, we have to picture all heated
and luminous bodies in a state of vibration, and the atoms of such
luminous bodies imparting the vibrations to the atoms of the Aether, in
the same way that the atoms of a bell impart their vibrations to the
atoms of the air in contact with it. These vibrations are then
propagated through the Aether in waves, which, entering the eye, impinge
or strike upon the retina at the back of the eye, and being transmitted
to the brain give rise to the sensation of sight. It must not be
forgotten that the waves of Aether, as pointed out in Art. 64 in
relation to heat, really form spherical shells which radiate out in all
directions from the central body which gives rise to them. Thus it can
be seen, that all points in the spherical wave which are at equal
distances from the vibratory or luminous body, must possess the same
intensity, and possess equal lighting powers. Light, therefore, like
heat, is due to a periodic wave motion set up in the Aether by the
vibrating atomic motion of heated or luminous bodies. It must be also
noticed, that if we could see the air through which the sound waves are
passing, we should see that each atom or particle of the atmosphere was
vibrating to and fro in the direction of propagation. If, however, we
could see an atom of Aether in vibration, accepting the principle that
Aether is atomic, we should see that each aetherial atom is not
vibrating in the direction of propagation, but across the line in which
the wave is travelling. Thus the vibration of the air is said to be
longitudinal, but the vibrations of the Aether are transversal. An
illustration of the transverse motion of a light wave may be obtained by
taking a rope and imparting to it a series of undulations by shaking it
up and down, when it will be observed that the wave motion of the rope
is transverse to the straight line in which it is propagated. The
physical explanation of the transverse vibration of light will be dealt
with in a subsequent article.
Now the question suggests itself to our mind, as to what effect the
atomicity of the Aether has upon the undulatory theory of light. Does it
establish it upon a firmer basis, or does it in any way destroy its
truth as a theory? I venture to think that the atomicity of the Aether
in no sense destroys any part of the undulatory theory of light, but
rather tends to confirm and establish it upon a logical and
philosophical basis.
For instance, as has been pointed out in Art. 47, in order for the
undulatory theory to have any existence at all, it is essential that the
Aether should possess the property of elasticity. But how the Aether
possessed the property of elasticity while at the same time it was
frictionless, and therefore possessed no mass, has been a problem that
has taxed the ingenuity and resources of scientists for a century past,
and up to the present is a problem which still remains unsolved. Now,
however, with our atomic Aether, it is just as easy to conceive Aether
transmitting a wave as it is for air to transmit sound waves, or water
to transmit water waves.
Tyndall, in his _Lectures on Light_, seems to have appreciated the
difficulty, and to avoid confusion, again and again refers to a
_particle_ of Aether. While Huyghens himself in speculating upon the
elasticity of the Aether in his _Traité de la Lumière_, 1678, makes a
suggestion as to its origin, which practically amounts to the fact that
the aetherial atom which gives rise to this elasticity is the core or
centre of a vortex ring. Thus it can be seen that the elasticity of the
Aether, so essential to the undulatory theory, is a problem that cannot
be solved apart from recognizing the hypothesis of an atomic Aether.
Then, again, in the undulatory theory of light, the density of the
Aether around molecules of bodies has to be taken into consideration to
account for such phenomena as the refraction and reflection of light,
but, as we have seen in Art. 46, such a property as density is
inconceivable in connection with a medium which is neither atomic and
possesses no mass. On the assumption, however, of an atomic and
gravitative Aether, the difficulty is at once solved, and the density of
the Aether, and different degrees of density are at once placed upon a
logical and philosophical basis. So that in relation to the elasticity
and density of the Aether upon which the transmission and reflection of
wave motion depend, an atomic and gravitative Aether establishes and
confirms the undulatory theory.
There is also another aspect of the subject that is worthy of notice. I
refer to the effect of an atomic and gravitative Aether upon Newton's
corpuscular theory of light. Newton's corpuscular theory failed in not
being able to account for the relative velocity of light in rare and
denser media, and if by an atomic Aether in conjunction with the
undulatory theory, the phenomenon can be accounted for, as I believe it
can, then our aetherial vortex atoms are analogous to Newton's
corpuscles. This distinction will, however, have to be made, viz. that
Newton supposed his luminous corpuscles to be emitted by the luminous
body, whereas in the conception of our aetherial atoms, we conceive them
to be stationary relatively in space, and only subject to those
vibrations and oscillations that give rise to the aetherial waves
recognized in the undulatory theory.
It would indeed be a consummation to be desired, if, by an atomic
Aether, it can be proved that Newton's Corpuscular Theory was made to
harmonize with the Undulatory Theory, and that it can be I am profoundly
convinced. Professor Preston is also of this view, for in his _Theory of
Light_, writing on this subject, he says, page 19: "In conclusion, we
may state that we believe an ingenious exponent of the emission theory,
by suitably framing his fundamental postulates, might fairly meet all
the objections that have been raised against it."
We will now apply the hypothesis of an atomic and gravitating Aether to
Huyghens' principle of wave propagation, and see if this atomicity in
any way destroys that principle, or whether it simplifies and confirms
it.
Let us briefly review our conception of the Aether before making the
application. In the first place, because Aether is gravitative, we
learned from Art. 45 that it surrounds all bodies in the universe, from
the smallest atom to the largest sun or star in the firmament of heaven.
Our sun, then, which is to our system the source of all its light, will
be surrounded by what are practically spherical aetherial envelopes or
shells which decrease in density as they recede from the sun (Art. 46).
These aetherial shells are, according to our conception, made up of
minute aetherial spherical vortex atoms possessing polarity and rotation
(Art. 43), and these atoms will be closer together the nearer they are
to the central body, because of the increased density of the Aether due
to the attractive influence of the sun. Thus, when a wave motion is set
up in the Aether around the sun by the intense atomic activity of that
incandescent body, each atom of that aetherial spherical shell or
envelope participates in the motion or impulse received, at one and the
same time, so that the wave is transmitted from envelope to envelope, by
the elasticity of the aetherial atoms which compose the envelope or
shell. Thus the light wave is always spherical in form, or nearly so, as
the rotational and orbital motion of the sun affect the exact shape of
the aetherial envelope as we shall learn more fully later on.
Further, the wave front always takes the form of a sphere, as the waves
are radiated out from the luminous body in all directions, and we shall
learn, in the next article, that the vibrations are always in the wave
front, that is, take place on the surface of each of these envelopes,
and these vibrations are also transverse to the propagation of the wave.
As these aetherial envelopes extend right into space, the wave is
transmitted from envelope to envelope by means of the aetherial atoms
with the velocity of 186,000 miles per second, but as each succeeding
envelope possesses a larger surface than the preceding one, the
intensity of the light is proportionally decreased. The surface of such
envelope is always proportionate to the square of the radius, the other
quantities remaining equal. So that the intensity of the light waves,
which are coincident with the surface of each spherical envelope, will
always vary inversely as the square of the distance from the luminous
body, which agrees with the law of inverse squares that governs light
and heat.
[Illustration: Fig: 4.]
We have considered the wave motion as a whole, that is, we have viewed
it from the standpoint of the whole of the aetherial elastic envelope.
Now we will look at the subject from the atomic standpoint, and see if
it is in accordance with Huyghens' principle of wave propagation.
We will suppose that an undulatory movement is started by a luminous
body at point _A_ situated in the Aether, and surrounded by that medium.
_A_ may represent a part of any luminous body, as the sun or star, while
_B_ _C_ and _B'_ _C'_ represent a segment of the aetherial envelopes
already referred to, which exist around the sun. We will further suppose
that the small dots surrounding the luminous body represent the
aetherial atoms forming the envelope, which transmit the impulse or
energy received from the atomic vibrations of the luminous body. As each
aetherial atom is moved or pushed forwards, each atom directly in
contact with it accepts and transmits the impulse. But each of these
atoms stands in relation to those in front of them, as they did in
relation to the first row of atoms, so to speak, and therefore exert a
corresponding impulse on the front row.
But the third row stands in relation to the fourth row as the second row
did to the third, and so on to infinity. Thus each atom being surrounded
by other atoms may be looked on as the centre of a new wave system, so
that every particle of the wave system is itself a centre of a new wave
system which is transmitted in all directions. As these innumerable and
minute wave systems co-operate with one another, they form a principal
wave system which is coincident with the surface of the spherical
envelope, part of which is represented by _B_ _C_. Then if we conceive of
all the aetherial atoms in part of the principal wave system _B_ _C_, as
themselves becoming the centre of wave propagation, by their wave
systems the principal wave will be transmitted further on into space to
another aetherial envelope _B'_ _C'_, which represents part of another
principal wave, which again is coincident with the surface of one of the
spherical aetherial envelopes. So that by the action of the aetherial
atoms which exist on all sides of the luminous body, the aetherial wave
can be transmitted from atom to atom in more or less spherical form.
Now let us compare this explanation of the transmission of light by an
atomic Aether with the celebrated Huyghens' principle which is thus
enunciated. "When an undulatory movement propagates itself through an
elastic medium, every _particle_ imitates the movement of the particle
first excited. But every particle stands in relation to the adjoining
ones in exactly the same relation that the first particle did to its
neighbours, and consequently must exert upon those surrounding it,
exactly the same influence as the first did. Every vibratory particle is
therefore to be regarded as if it were the originally excited particle
of the wave system; and as the innumerable and simultaneous elementary
wave systems co-operate with one another at each instant, we obtain
exactly that principal wave system by which the elastic medium appears
at any moment to be moved." Now here, in this statement, we have the
definite term _particles_ used several times by Huyghens. But in the
generally accepted theory of the Aether, such a term is unknown and
unrecognized, with the obvious result that the definite and simple
statement of Huyghens loses all its simplicity and meaning. Replace,
however, the non-atomic Aether as at present recognized, by an atomic
and gravitating Aether, and then Huyghens' exposition or principle
stands out in all its simplicity and clearness, and finds in an atomic
Aether its literal fulfilment and complete verification.
In conclusion on this point, viz. that light is a mode of aetherial
motion, let us endeavour to form a mental picture of our atomic and
aetherial world. We have to remember that every particle and atom of
matter in existence are ever vibrating, and by their vibrations are ever
creating and generating Aether waves in the aetherial medium. These
waves, begetting others, the process is continued until they are either
intercepted and brought to rest by other matter, or else speed away
until they reach the boundary of space.
Now it is scarcely necessary for me to say, that if one atom can create
and generate these Aether waves, a thousand atoms can create them in
greater abundance still, and millions of atoms in even still greater
abundance, and so on in proportion to the quantity or bulk of the matter
vibrating. Further, as it is with quantity, so will it be with
intensity, or activity of vibration. The more intensely an atom
vibrates, the more intense would be the movement of the generated Aether
waves, and the intensity would be in exact proportion to the intensity
of the motion of the atoms vibrating. In regard to the power of atomic
motions or vibrations, those are the greatest and most intense in energy
or motion, which are produced by combustion or burning. The chemical
activity by which the burning is brought about arouses and excites the
atoms of matter subject thereto, into an intensity of motion, thousands,
it may be millions of times greater than can be produced by any other
known means. Therefore it can be readily seen, that the Aether waves
generated by this means will be greater and more abundant, both in their
volume and intensity, than the Aether waves produced merely by a cold
body. For example, take a candle at night-time when the light has
disappeared; look at it and feel it. Though its atoms are all in motion,
generating Aether waves which are impressed with its own particular form
and colour, yet it can scarcely be seen even at a short distance; but
light it, and what a change takes place! We can both see it, and are
enabled by its light to see other things also. By the power of
combustion, its atoms have been excited into greater energy or motion,
generating and speeding Aether waves on every side, and these Aether
waves being reflected and re-reflected by the atoms of the air, and the
walls of the house, give light to all that are in the house. I must now
ask the reader to refer to Art. 64 on Radiant Heat, in order that we may
recall facts regarding the heat of the sun. Remembering the intensity
of the heat of the sun as calculated by Herschel and others, and
remembering that the sun is 1,200,000 times larger in volume than our
earth, the question naturally suggests itself to our mind, what must be
the volume and intensity of the light waves as they flow from the sun
into space? What a storm of fury and of motion must there be within the
aetherial atmosphere around the sun, and with what volume and power must
these light waves speed away from so mighty a source! Some idea may be
gained from the fact that they speed away to the distant Neptune, a
distance of nearly three thousand millions of miles, and impart to that
planet the energy of light and heat which to the planet forms the
physical source of all its life and activities. Thus from the sun, the
centre of the solar system, there are ever being poured forth into space
these aetherial light waves. The solar fires are ever glowing, and their
flames ever burning, robing the solar disc with its quivering fringe, or
madly leaping on every side to a distance of one hundred thousand miles,
and by their madness lashing the aetherial atmosphere into fury,
creating aetherial waves, myriads upon myriads, and sending them with
lightning speed across the intervening space. As swift-footed messengers
they come, the bearers of life and beauty to distant planets. They come
to this our island home in space, these aetherial light waves, like rich
argosies freighted with the treasures of light, of life, of beauty, and
of glory, and the transmission of this life and beauty is effected by
the incessant wave motion generated in the Aether by the central body of
our solar system, the sun. Let us therefore endeavour to form a mental
picture of this aetherial wave motion with its transverse vibrations.
ART. 71. _Transverse Vibration of Light._--In the previous article we
saw that the vibration of light was transverse to the line of
propagation. If we could see the particles of air which are vibrating
when sound waves are produced, we should find that each particle or atom
is vibrating backwards and forwards in the direction of propagation.
In the case of an aetherial atom, however, which, according to our own
theory, participates in the vibration, we have to try to conceive of
each atom as vibrating across the line of propagation. So that if _A_
_B_ represents a ray of light proceeding from a luminous body, as the
sun (Fig. 5), then the vibration must be across the line, as up and down
and across that line as shown in the figure, each phase of the vibration
being at right angles to the line of propagation--that is, to _A_ _B_.
How can we form a physical conception of this phenomenon? There must be
some physical explanation to it, for if it be an effect there must be a
cause for its existence and production. Up to the present, however, no
physical explanation has been forthcoming, so that for over 200 years a
frictionless medium has failed to account for, or to explain, the
transverse vibration of light as suggested by Fresnel.
If, therefore, by the hypothesis of an atomic and gravitative Aether, we
succeed in accomplishing a result that a frictionless Aether has failed
to accomplish, then the explanation will be a most important factor in
proving the atomicity and consequent gravitative property of the Aether.
Let us therefore revert to our hypothesis of the Aether as given in Art.
45. From that we learn, because Aether is atomic, it is also
gravitative, and therefore forms around every atom and molecule, every
satellite, planet, sun and star, an aetherial atmosphere--such aetherial
atmosphere being doubtless proportionate to the mass of the atom or
molecule or planet as the case may be, in accordance with the Law of
Gravitation. We shall consider this view of the subject later on.
[Illustration: Fig: 5.]
Thus we learn that every particle of matter, and every body in the
universe has its aetherial atmosphere so to speak, to which it is held
bound by the universal Law of Gravitation. In the case of a satellite
or planet or sun or star, that atmosphere will be more or less spherical
in shape, decreasing in density as it recedes from the attracting body.
As we saw in the previous chapter, Tyndall stated that the waves of
light really formed spherical shells which surrounded the luminous body.
In the conception of an atomic and gravitating Aether we can form a
physical conception of these aetherial shells, which can be pictured as
elastic envelopes, or rather series of envelopes surrounding each
particle of matter, and also surrounding each satellite, planet, sun,
and star; each envelope getting gradually less and less dense as the
distance from the central body is increased.
Now we learn from experiments that the vibration is always in the wave
front, but the wave front is coincident with the surface of each
aetherial spherical shell, therefore the vibration must be in, and
coincide with, the surfaces of the spherical shells that are formed
around every body in the universe.
We are now, however, dealing specially with one body which is the source
of light, viz. the sun, and have therefore to picture the sun as being
surrounded by these aetherial elastic envelopes, which gradually get
less and less dense as they recede from it. What, therefore, will be the
effect of the heat of that body as it is poured forth into space? We
have already learned (Art. 63) of the untold quantity of heat that is
continually being poured forth into space from the sun with its diameter
of 856,000 miles, and its circumference of over 2-1/2 million miles.
What intense activity it must generate in the Aether near its surface!
and what must be the direct effect of that heat upon the aetherial
elastic envelopes or shells which surround it?
Perhaps the answer can be best illustrated by a simple experiment. Let
us take an ordinary toy balloon, with its elastic envelope, and fill it
moderately full with air, and observe what the effect on it is when we
put it near the fire. Gradually, as heat is imparted to the air in the
balloon, the air which is also elastic expands, with the result that the
envelope of the balloon is extended, and its size enlarged. Now withdraw
it from the fire and note what happens.
As the air inside gets cold again, the elastic envelope of the balloon
gradually shrinks, until it has been reduced to its former size. What
has been taking place during this experiment with regard to the elastic
envelope and the atoms thereof? May we not say that there has been a
vibration or oscillation, among the particles which go to make up the
elastic envelope, that forms the surface of the balloon? Certainly there
has been some form of motion, and that motion took first the form of an
expansion, and then contraction of the individual particles; and we have
only to conceive of this process being repeated quickly and
continuously, to form a mental picture of what takes place in any
aetherial elastic envelope or shell that surrounds the sun.
The illustration is not, however, perfect, because we have made the
source of heat to be outside instead of inside the elastic envelope, as
is the case with the sun and its aetherial atmosphere or envelope. We
will therefore slightly modify the experiment, and take two balloons,
_A_, _B_, one smaller than the other, and put the smaller one _A_ into
the interior of the larger one, inflating the smaller one, so that it
can be situated in the middle of the larger one, the latter having twice
the diameter of the smaller one, as in the diagram (Fig. 6). To the neck
of the smaller balloon _A_ we will attach an india-rubber tube which
ends in a closed bulb _C_. We have now the two balloons inflated. Let us
press the bulb _C_ and notice what happens. The effect will be exactly
the same as it was when we brought the balloon in contact with the heat
of the fire in the first experiment--that is, the elastic envelope will
be again expanded. As soon as we take the pressure from the bulb _C_ the
envelope, being elastic, seeks to recover its original position, with
the result that it springs back to its original size. If we pressed the
bulb _C_ 20 times per minute, we should get 20 vibrations of the
particles of the envelopes per minute, and if we pressed it 1000 times
per minute, we should get 1000 vibrations among the particles of the
elastic envelope, so that the number of vibrations would correspond to
the number of times we pressed the bulb. Now how did this vibration
reach the elastic envelope of the balloon _B_ from the balloon _A_?
[Illustration: Fig: 6.]
The reply is, by means of the particles, or atoms of air that exist
between the two surfaces of the balloons, and that transmission would
take the form of a wave propagated from particle to particle, so that we
might put dots on the right side of _A_ to represent the atoms of air
which transmit the wave from _A_ to _B_.
But the vibration which takes place in the surface of the envelope of
the outer balloon is _across_ this line of propagation, because as the
wave proceeds from _A_ to _B_, the elastic envelope expands and
stretches always _across_ the line of propagation--that is, it stretches
up and down, left and right, as it is expanded outwards, so that the
vibration or oscillation of the particles always takes place in the
surface of the elastic envelope across the line of propagation. Let us
therefore apply the result of this simple experiment to our solar system
and the Aether, and see if it can be made to explain the transverse
vibration of light. Let _A_ represent the sun (Fig. 7) and _B_ an
aetherial elastic envelope surrounding the sun. In this case we dispense
with the bulb _C_, as the sun possesses within itself the power to
generate heat, and so to produce the required expansion of the elastic
aetherial envelopes _B_, _G_, _H_, etc.
[Illustration: Fig: 7.]
Instead, however, of having air particles between _A_ and _B_, we will
put in their place our aetherial atoms which we have conceived according
to Art. 44. These surround the sun, represented by _A_, forming elastic
spherical shells or envelopes. As the sun radiates its heat into space,
it urges the aetherial atoms against each other, with the result that
they transmit the energy from atom to atom, or particle to particle,
till they come to the elastic aetherial envelopes of _H_, _G_, _B_.
The effect on _B_, or on any other aetherial envelope, is to expand it
outwardly, and thus set the atoms of which it is composed into
vibration. The wave, which is now an aetherial wave travelling with a
velocity of 186,000 miles per second, may be represented by the line _D_
_E_. But while it is travelling from _D_ to _E_ the same energy is being
radiated out in all directions, so that a wave reaches the whole surface
of the elastic envelope _B_ at the same time, with the result that the
whole of the shell or envelope is set in vibration as it expands
outwardly.
Thus the vibration is always in the wave front, and the wave front is
always coincident with the surface of one of these envelopes, and as
these aetherial envelopes are themselves formed by aetherial atoms, the
wave is spread outwardly from any central point in a spherical form as
proved by experiment. Not only, therefore, is the vibration in the wave
front, but it is always transverse to the line of propagation, for the
simple reason that the surface of the spherical shell or envelope is
always at right angles to the radius vector or straight line which joins
any centre to the surface of a spherical envelope.
As soon as the aetherial atom which forms the spherical aetherial
envelope has reached the limit of its expansion, it seeks to recover its
former position because of its elasticity, with the result that the
whole envelope contracts again, and arrives at its original position in
space ready to accept motion again and transmit it onwards in the same
manner as before.
Thus, by the acceptance of an atomic and gravitating Aether, we may form
a physical conception of one of the greatest problems in optical
phenomena, viz. the transverse vibration of light which always takes
place in the wave front, and across the line of propagation. Whether
this explanation is exactly correct in detail, or not, I am convinced
that the true physical explanation of the problem is to be found in an
atomic and gravitating Aether, as hitherto a frictionless Aether has
failed even to suggest to any scientist how such a transverse vibration
can take place.
ART. 72. _Reflection and Refraction._--A ray or wave of light is said to
be reflected when it meets with an obstacle which opposes its free
passage and turns it back. We have illustrations of this law of
reflection in the case of water waves striking against a breakwater, or
a sound wave striking against the wall of a room. In either case the
wave is turned back, and reflection is the result. A ray or a wave of
light is said to be refracted when, in passing from one medium into
another, it is turned from the straight path in which it was going
before it entered the refracting medium. An illustration of the
refraction of light is to be found in the case of the glass lens, so
often used to converge the light waves into one focus. We have up to the
present dealt with only two theories of light, the Corpuscular theory
and the Undulatory or Wave theory. We have seen how both harmonize with
Huyghens' principle, and the question arises as to whether both can be
made to harmonize with the phenomena of reflection and refraction.
In the Corpuscular theory we have luminous particles emitted by luminous
bodies. These particles we have learned are practically synonymous with
our aetherial atoms.
In the Wave theory it is impossible to conceive of a wave without
conceiving of particles which transmit the wave; even Huyghens refers to
particles of Aether, and so does Tyndall in his _Notes on Light_.
In the Electro-magnetic theory of light we have again to think of atoms,
which are termed electrons by Dr. Larmor and Sir William Crookes; while
Professor J. J. Thompson calls them corpuscles.
So that in all three theories we have the same fundamental idea of
atoms, either expressed or imagined, underlying all the three theories.
Now what is the property of the Aether on which all reflection and
refraction is based? Is it not the property of density? Fresnel assumes
that reflection and refraction of light are dependent upon different
degrees of density of the Aether associated with any body, and has given
a mathematical formula, which decides the index of refraction, such
formula being entirely dependent upon the relative density of the Aether
in association with the refracting medium.
But with a frictionless medium, it is an absolute impossibility to
conceive of different degrees of density of the Aether in association
with matter.
If the Aether does possess different degrees of density which decide the
refractive index of the substance, then of a certainty there must be
some law to govern and decide the density, and that law can only be the
Law of Gravitation.
As Young points out in his Fourth Hypothesis, every particle of matter
has an attraction for the Aether by which it is accumulated around it
with greater density. Now on the basis of our conception of a
gravitative Aether, every atom and molecule, and indeed every body in
the universe, possess aetherial atmospheres, which possess varying
degrees of density, the denser layers being nearest to the nucleus of
the atom or molecule as the case may be, the elasticity of each layer or
envelope being always proportionate to its density.
When we apply the corpuscular theory to the reflection of light we find
that it satisfactorily accounts for the phenomenon.
According to Newton's corpuscular theory, each luminous particle travels
in a straight line through a homogeneous medium. When, however, it comes
almost into contact with a reflecting surface, which in our case we
conceive to be a layer of one of the aetherial elastic envelopes
surrounding the atoms or molecules of the reflecting body, then,
according to Newton, the light particle is repelled, or reflected by the
medium; the angle of reflection or repulsion being always equal to the
angle of incidence. So that the emission theory harmonizes with the wave
theory in regard to reflection.
When, however, we come to deal with the refraction of light, the
corpuscular theory apparently breaks down, and it was in relation to
this phase of the phenomena of light that the undulatory theory
overthrew the corpuscular theory.
According to the corpuscular theory, when a luminous particle or
corpuscle is nearing the surface of a denser medium, as glass or water,
it was attracted by the denser medium, with the result that the velocity
of the particle in the denser medium was greater than its velocity in
air. But direct experiments prove exactly the opposite, as it is found
that when light passes from a rare into a denser medium, the velocity of
light in the denser or more refracting medium is less than it was in the
air. Here then was a test to decide the respective merits of the two
theories. As the undulatory theory was able to give a satisfactory
explanation of the phenomenon, the corpuscular theory was rejected, and
the undulatory theory was accepted. Now the question suggests itself, as
to whether it is possible to reconcile the two theories in relation to
the refraction of light by our conception of an atomic and gravitative
Aether. I believe it is possible. Let us look at the case for a moment.
We have, according to our theory of the Aether, to conceive of all atoms
and molecules, of all planets and suns and stars, being surrounded by
aetherial elastic atmospheres, or envelopes, which, like the atmosphere
in association with the earth, are always the densest nearest the
nucleus of the atom, getting gradually less and less dense the further
they recede from the central point. Further, according to our theory,
with regard to the elasticity or pressure of these elastic envelopes,
they exert a pressure proportionate to their density. So that the nearer
the aetherial atmosphere or envelope is to the central point or nucleus
of the atom, the greater will be the elasticity or pressure.
Now what I wish to call the reader's special attention to is, that the
pressure in each and every case of the aetherial elastic envelopes which
surround the central nucleus, is always directed _away from_ the central
point, and here it seems to me is the solution of the difficulty which
Newton failed to solve. For when a luminous corpuscle enters any medium,
assuming it to do so, it would have to overcome the pressure due to the
increased elasticity of the denser aetherial envelopes, and as the two
motions, viz. that of the incident ray, and the pressure due to the
elasticity of the elastic envelope, would be in opposition to each
other, the result would be that the luminous corpuscle, if it entered
the medium at all, would be retarded and not accelerated as suggested by
Newton, and such a result is perfectly in harmony with experiment. So
that by our theory of an atomic and gravitating Aether, it seems to me
that it now becomes possible to reconcile the two theories.
There is another difficulty that the emission theory had to contend
with, and that was, how was it possible for the same surface of any
substance to reflect and refract a corpuscle at one and the same time?
Newton overcame this difficulty by suggesting, from the results of his
observations on certain coloured rings, that each particle had what he
called certain phases or fits, of easy reflection or refraction, so that
at certain times they would be refracted, and at other times they would
be reflected.
Boscovitch has suggested that the fits were due to the fact that each
luminous corpuscle possessed polarity; which, by rotating, alternately
offered their different sides to the refractive and reflecting surfaces,
so that sometimes they would be reflected or repelled, and at other
times attracted or refracted.
A similar hypothesis has also been suggested by Biot. Now if such a
hypothesis will satisfactorily account for the fact that the same medium
will reflect or refract the luminous corpuscles, as the case may be,
then in our aetherial atom we have the very conditions which would
satisfy both Boscovitch and Biot's hypothesis. For one of the properties
that we suggested regarding our aetherial atom was, that it possessed
rotation like our own earth, and that it also possessed polarity.
The harmonizing of the two theories, therefore, seems to rest upon the
atomicity or non-atomicity of the Aether.
It is absolutely certain that the electro-magnetic theory of light
demands the recognition of some form of atomicity for the Aether. For if
light be really an electro-magnetic phenomena, as has been proved by
Maxwell and experimentally demonstrated by Hertz, then, in view of the
fact that the atomicity of electricity is coming within the scope of
direct experiment as asserted by Dr. Larmor, unless we accept atomicity
of the Aether in some way, we shall be in the unphilosophical position
of having the Aether of space not being composed of atoms, while the
electricity associated with that Aether in some unknown way is composed
of atoms. In other words, we shall have a non-atomic body composed of
atoms, which conclusion is absurd. Therefore, from the electro-magnetic
theory of light, we are again compelled to postulate atoms of some kind
for the Aether.
If there are electrical atoms in association with the Aether, then they
must be of two kinds, positive and negative, as it is impossible to find
positive electricity disassociated from negative. Therefore, from the
electro-magnetic theory of light we get further evidence of the polarity
of the aetherial atom, by which Newton's fits of easy reflection or
refraction may be physically conceived.
I am convinced, that with the hypothesis of an atomic and gravitative
Aether as suggested by Young in his Fourth Hypothesis, all three
theories of light in relation to the phenomena of reflection and
refraction can be harmonized. I wish only to point out the direction in
which to look for the solution, and must leave it to scientists to work
out the problem.
ART. 73. _The Solar Spectrum._--When a ray or beam of solar light is
passed through a prism, it is broken up or decomposed into its
constituent parts. This is called dispersion, and conclusively proves
that the light from the sun is not a simple, but a compound colour. We
have illustrations of this decomposition of pure white light in the
rainbow, where the colours of the sunlight are revealed against the sky
with clearness and precision. A simple experiment to prove that the
solar light is a compound one may be made by boring a small hole in a
shutter, and then allowing the sunlight that passes through the hole to
fall upon a prism, such as the pendant of a candelabrum. When this is
done, then on the opposite wall of the room will be seen, not one
colour, but seven colours, ranged in the following order: Red, Orange,
Yellow, Green, Blue, Indigo, Violet. This is termed the Visible
Spectrum.
It may be asked, What is the cause of the various colours in the
spectrum? We have already seen that light is due to a wave motion of the
Aether, and it can be demonstrated that the various colours of light are
due to different wave lengths. Colour is to light what pitch is to
sound. As has been shown in Art. 62, the pitch of a note depends upon
the number of air waves which strike upon the tympanum of the ear in a
given time. The more rapid the vibration, the higher the note. The more
rapidly a sounding body vibrates, the shorter will be the length of each
wave. If a violinist wants to produce a note of higher pitch, he presses
his finger on the string, thereby shortening it, and by so doing
increases the rapidity of vibration, and raises the pitch of the note.
Now the colours of the spectrum are to the eye what the notes are to the
ear. The aetherial waves which produce the red colour are slower in
their vibrations, and are longer than those which produce the orange
colour. Those which produce the orange colour are of slower vibrations,
and longer than those which produce the yellow colour, and so on through
all the other colours; until we get to the violet and to the
ultra-violet, or invisible violet rays, which are the most rapid in
their vibrations, and consequently their wave lengths are the shortest
of the whole group. It has been ascertained that it takes about 39,000
waves of red light to measure an inch if placed end to end. Now light
has a velocity of 186,000 miles per second. If this is reduced to
inches, we find that there are 11,784,960,000 in that distance. Let us
therefore multiply this number by 39,000, and we shall then find how
many waves of red light must enter the eye to produce the sensation of
red colour. That number is 459,613,440,000,000, so that all these waves
enter the eye in one second of time, and must strike the retina of the
eye in order to produce the sensation of redness. In the same way, the
number of waves that must strike the retina of the eye to produce the
sensation of violet can be determined. It takes about 57,500 waves of
violet to measure an inch, so that a violet wave is only 1/57000 part of
an inch in length. All the other colours of the spectrum which lie
between the violet and the red waves gradually get longer and longer in
their wave lengths, and slower and slower in their vibrations, until at
the red end of the spectrum and beyond it we have the longest waves,
which are from 1/39000 part of an inch in length to 1/10000 part of an
inch.
The seven colours seen in the spectrum are called the Visible Spectrum.
There are, however, rays of light beyond both ends of the spectrum which
do not affect the optic nerves of the eye, and therefore are invisible
to sight. The rays in the spectrum which lie beyond the red are termed
ultra-red rays, while those beyond the violet are called ultra-violet
rays. It can be proved the former are rich in heating power, while the
latter possess great chemical power. By means of an instrument known as
the thermo-electric pile, or thermopile, the various heating power of
the whole spectrum, visible and invisible, can be determined.
Let us look for a moment at these invisible or dark rays. Strictly
speaking, all light is invisible, as we cannot see light itself, we can
only see it by reflection. We have seen that light is due to a wave
motion in the Aether, but we cannot see that wave motion, neither can we
see the Aether itself, so that it is not strictly correct to call a ray
visible or invisible. We have, however, accepted the terms in relation
to the rays of the spectrum, to distinguish between the invisible or
obscure rays of the spectrum and the visible rays. It was Sir W.
Herschel who first discovered the existence of these invisible waves. He
passed a thermometer through the various colours of the solar spectrum,
and then noted the temperature of each colour. He did not, however, stop
at the limit of the visible spectrum, but experimented with his
thermometer beyond its limits, and then found that beyond the red rays
there were other rays, the ultra-red rays, which possessed greater
heating power than any other rays of the spectrum. Thus his experiments
proved, that side by side with the luminous or light waves, there were
other rays, which, though they possessed greater heating power, yet were
not able to excite the optic nerve, and so produce the sensation of
sight.
From these facts we learn that the solar spectrum may be divided into
three parts--
1. The red or ultra-red end of the spectrum which possesses the greatest
heating power.
2. The central part, yellow and green, which is the greatest in luminous
power or light waves.
3. The violet or ultra-violet end, which possesses great chemical or
actinic power as it is sometimes termed.
We have already seen (Art. 69) that the same aetherial waves which give
rise to heat, also give rise to light, and that the only physical
difference between heat and light is, that the waves which cause the
phenomena of heat are of slower period, and of greater length, than
those which cause the phenomena of light. From the solar spectrum we
learn that there are a third class of Aether waves, which are of more
rapid vibration, and therefore shorter in length than either the
aetherial heat waves or the aetherial light waves. As already stated,
these are called chemical or actinic waves, because they possess a
greater chemical power than either the heat or the light waves that form
the central part of the spectrum.
Now this question suggests itself to us in relation to these chemical
waves. What are these so-called chemical waves that are produced in the
aetherial medium by the activity and heat of the sun? It must be
remembered that the aetherial waves which give rise to both light and
heat, and also these chemical waves, are first set in motion by the sun,
at least as far as our solar system is concerned. We are perfectly
conversant with the phenomena and characteristics of both heat and
light. We are able to exactly determine what their particular effect
will be on matter, and to describe that effect in a perfectly
straightforward manner. The same, however, cannot be said of these
so-called chemical waves that lie chiefly in the violet and ultra-violet
end of the solar spectrum. What, then, is a chemical wave, its
particular nature, and its exact properties? That we know it can
decompose certain compounds, as Carbonic Acid Gas, CO_2, and so give
rise to chemical decomposition, has been proved by Professor Tyndall and
others, but I have never yet seen any record of any attempt to find out
what these chemical waves are. There may have been such attempts made to
discover their origin and character, but I have not seen any such
record. I purpose, therefore, to offer an explanation as to the
character and origin of these chemical or actinic waves, which I hope to
prove by philosophical reasoning. We have already seen (Arts. 54 and 59)
that both heat and light are convertible, or can be transformed into
electricity, so that the same aetherial wave motion which can produce
light can also produce heat, and that in its turn can produce
electricity. Thus we learn that there is a very close identity between
light, heat, and electricity; indeed it can be demonstrated that the
same aetherial wave motion which produces electricity can produce the
other two.
Lorentz,[14] in an article on "The Identity of Light Vibrations with
Electric Currents," states that "the vibrations of light are themselves
electric currents." Now if this is true, and I believe it to be true, as
I hope to prove later on from Clerk Maxwell's works, then it necessarily
follows, that wherever we get aetherial light waves, we must at the
same time also get aetherial electric waves. If that be so, then in the
solar spectrum we ought to have revealed to us, not only indications of
the presence of the heat and light vibrations, but equally so the
presence of electric waves. This, I believe, is actually the case, and
the electric waves are the so-called chemical waves in the violet and
ultra-violet end of the spectrum. I think that we shall find sufficient
arguments and analogy to support this hypothesis, as we look further
into the matter. One of the greatest scientists of the past century,
Clerk Maxwell, has given to the world the genesis of what he termed the
Electro-Magnetic Theory of Light, in which he proved that light was
nothing more nor less than an electro-magnetic phenomenon. He pointed out
that the same Aether which was concerned in the propagation of light and
heat through space, must therefore be equally concerned in the
propagation of electric displacements in the free Aether; as he states,
it would be philosophically wrong to assume that there was one aetherial
medium for light, and another for electric phenomena. If, therefore,
there is such a theory as the Electro-Magnetic Theory of Light, and
there undoubtedly is, as has been proved by the researches of Hertz on
electric waves, then it follows, either that light waves are themselves
electric currents, as suggested by Lorentz, or that the light waves are
directly associated with electric waves in the same way that they are
associated with heat waves. So that the only difference between them
would be one of period of vibration and of length, the electric waves of
the Aether being of greater rapidity and therefore of shorter length
than either the light or heat waves. The only conclusion, therefore,
that it seems possible to come to regarding these chemical waves is,
that they are the electric waves of the spectrum. Thus, in the solar
spectrum, there are three classes of waves indicated by the various
colours, and beyond the limits of these colours, viz. (1) Thermal or
Heat waves in the red or ultra-red end of the spectrum; (2) Luminous or
Light waves at the middle of the spectrum; and (3) Electric or Chemical
waves in the violet or ultra-violet end of the spectrum. Now in looking
at this hypothesis from the standpoint of our Rules of Philosophy, I
venture to assert that all the three rules are satisfactorily fulfilled,
and that being so, the hypothesis advanced is philosophically correct.
In the first place, such a conception that the chemical waves or violet
waves are really electric waves is simple in its hypothesis, and so
fulfils our first Rule of Philosophy. It is simple, because it puts in
the place of unknown chemical waves, a certain kind of aetherial waves
with whose action we are definitely familiar, and whose origin and
effect can be satisfactorily accounted for, as proved by Hertz. Chemical
waves are not simple in conception, because we do not know exactly what
they are, or how they are originated. Besides, as Newton points out,
there is nothing superfluous in Nature. If one cause can effect the
desired end, as electric waves, then another cause as chemical waves is
superfluous and unnecessary. Further, in our hypothesis of the electric
character of these chemical waves, we have a solution which
satisfactorily fulfils the second Rule of our Philosophy. Experience and
experiment teach us, that there are electric waves constantly being
generated in a thousand ways. Indeed, it is an absolute impossibility to
perform the simplest act of ordinary life, as brushing a hat, or wiping
the boots on a mat, cutting an orange, or any other act of simple
everyday life, but that these aetherial electric waves are generated.
But as for these so-called chemical waves, experience has little to say
about them, and experiment still less. If we decompose water, dividing
it up into two gases, Oxygen and Hydrogen, we do it by passing a current
of electricity through the water. If we want to decompose or split up a
binary compound, as HCl, into its two elements, Hydrogen and Chlorine,
then we can do it by electricity--that is, by the decomposing action of
these electric waves. In all these experiments and results we know
definitely what we are doing, and what the effect will be. There is no
vagueness about the terms used. When we speak of chemical action we look
to a definite source for that action, and we do not say that such action
is produced by chemical waves, but rather by electricity. So that all
experience teaches us, and all experiments made by such men as Faraday,
Davy, Maxwell, and Hertz confirm the statement, that these aetherial
electric currents can accomplish all that the so-called chemical waves
accomplish, and that being so, the third Rule of our Philosophy is also
fulfilled, as we have in the aetherial electric waves a satisfactory
explanation for the fact which we seek to explain, viz. the character
and origin of the chemical waves that exist in the violet end of the
spectrum. Thus, we learn, that not only is the sun the source of all
heat and light, in that it gives rise to the vibrations of the Aether
which are propagated through it in waves, but that it is also the source
of all electric waves in the solar system, in that electric currents are
primarily due to the wave motion set up in the Aether, those electric
waves also traversing space with the velocity of light.
[Footnote 14: _Phil. Mag._, 1867.]
ART. 74. _Direction of Ray of Light._--In Art. 65 it was shown that the
direction of a ray of heat was that of a straight line from the heated
or luminous body from which the Aether waves proceeded. We have also
seen in Art. 69 that the aetherial waves which give rise to the
phenomena of heat are identical with those that give rise to light, so
the direction of a ray of light must also be that of a straight line
proceeding from the luminous body. A ray of light is a line
perpendicular to the Aether waves which are propagated through space in
concentric spheres from the luminous body, which, by its atomic
vibrations, gives rise to the light waves. It must, however, be
remembered that rays have no physical existence, for it is the waves
that are propagated, and not the ray, which simply indicates the
direction that the light takes, this truth being known as the
rectilineal propagation of light. That light proceeds in straight lines
may be proved in several ways. For example, we cannot see round corners,
which would be possible if light took a curved path instead of a
straight one. A better proof, however, may be obtained by making a small
hole in the window-shutter, and allowing the sunlight to pass into the
darkened room. The beam of light which passes into the room will then be
seen to take a straight course, its presence being revealed by the
particles of dust that float about the room.
Another conclusive proof that light proceeds in straight lines is to be
found in the fact, that all images formed on any screen by the rays of
light after passing through a small hole are inverted. For example,
suppose we have a window-shutter with a small hole in it, while in the
garden fronting the window there stands a tree. Now if the rays of light
which pass from the tree through the hole in the window-shutter are
allowed to fall upon a screen in the darkened room, it will be found
that the image is inverted.
This is accounted for by the fact, that the rays cross each other at the
hole, and proceeding in straight lines, form an inverted picture on the
screen. It can further be proved, that the path of a ray of light
through space as it proceeds from the sun is also that of a straight
line. Whenever there is a solar eclipse we have light so long as we can
see the smallest part of the sun's surface. The instant, however, that
we have a total eclipse, at that instant the whole of the light of the
sun is shut off, and for a brief space there is darkness, until the
planet which is causing the eclipse has passed on in its orbit and the
sun's surface reappears again. Now if light did not proceed in straight
lines, such an event as a total eclipse would be impossible; because, if
the light proceeded in curved lines instead of straight ones from the
sun, then even when the planet which causes the eclipse got directly
between the earth and the sun, the rays of light being curved instead
of straight would bend round the eclipsing planet, and so would not all
be intercepted, and thus such an event as a total eclipse would be an
impossibility. From this we learn, therefore, that the path of a ray of
light as it proceeds from the sun through space is that of a straight
line, and that the path corresponds to the radius vector of a circle,
which is also the path that the centripetal force takes.
Viewing the matter from the standpoint of the solar system, we find the
sun, which is the centre of that system, exerting an attractive force
along the radius vector of all the orbits of the planets, with a force
which decreases in intensity inversely as the square of the distance. At
the same time, being the source of all light, it is constantly
propagating into space aetherial light waves with a velocity almost
inconceivable; which also decrease in exactly the same ratio as the
attractive power of the sun decreases. If, therefore, it can be shown
that there is such a truth as the dynamical value of light, in the same
way that it has been shown that there is a dynamical value of heat, then
it follows, that not only is the sun the centre of an attractive power
which proceeds in straight lines, but it is equally the centre of a
power whose influence and motion are exerted along exactly the same path
as the centripetal force, but in an opposite direction, that is, away
from the sun. I hope to be able to show that the aetherial light waves
do possess such a dynamical value, and if that is accomplished, then not
only from the realm of heat, but also from the realm of light, we shall
have conclusive evidence of a power or motion whose influence is
directed away from the sun, which, therefore, would correspond to a
centrifugal force--that is, a force or motion directed from a central
body as the sun.
ART. 75. _Intensity of Light._--The intensity of light diminishes with
the distance from the luminous body, according to the same law that
governs sound, and heat, and electricity. We have already seen (Art. 67)
that the intensity of heat diminishes inversely as the square of the
distance, so that if the same law holds good for light that holds good
for heat, then, according to the law of the inverse squares, if we
double the distance from the luminous body, the intensity of light is
only 1/4 of what it was in its first position. If the distance be
trebled, then the intensity will be decreased 1/9. This can easily be
proved by the following experiment: Suppose we have a lighted lamp, and
at a distance of 1, 2 and 3 feet respectively, we have three square
surfaces. It can then be demonstrated that the light which falls on the
square 1 foot away, if allowed to fall upon the square 2 feet away,
would cover four times the area of the first square; and if allowed to
fall on the square 3 feet away, it would cover nine times the area.
Therefore the intensity of the light on the square 2 feet away, covering
four times the area, would only be 1/4 of what it is on the square 1
foot away, while the intensity of light on the square 3 feet away, which
covers nine times the area, would only be 1/9 of the intensity received
by the first square.
If the difference in the distances therefore be represented by the
figures 1, 2, and 3 feet respectively, the intensity would be
represented by the figures 1, 1/4, 1/9. The decrease in the intensity of
light is really a decrease in motion. The intensity of a note in sound
depends upon the vibration of the particles of air, while the intensity
of light also depends upon the vibrations of the aetherial atom.
If, therefore, we get a decrease in the vibration of the aetherial atom,
the further we get from the luminous body, it can be readily seen that
the intensity of light really implies a decrease of motion.
Now let us apply the law of inverse squares in relation to light to the
solar system. We have the sun with its huge form all aglow with fires,
as the source of all light to the planetary worlds that revolve around
it. Year in and year out, for many ages past, the sun has been pouring
out its light into space on every side, lighting up the planets or other
bodies that revolve round it on that side only which is presented to the
sun. Thus Mercury, at its distance of about 36,000,000 miles, obtains a
light from the sun which is of far greater intensity than the light
which Venus receives, while Venus receives a light of greater intensity
than the light which the Earth receives, and the Earth receives light of
greater intensity than any of the planets outside its orbit in the solar
system, as Mars, Jupiter, Saturn, Uranus, or Neptune. This decrease in
the intensity of light is according to the inverse square of the
distance from the central body, the sun. So that if we have one planet
at twice the distance from the sun, as compared with another planet, the
intensity of light at that distance will be only 1/4 of the intensity
received by the nearer planet. This decrease of the intensity of light,
however, may be compensated for by a difference in the constituents of
the planets' atmosphere, by means of which it may be possible that the
outermost planets enjoy climatic conditions similar to our own.
Now we have proved, in the previous chapter, that heat is a repulsive
motion, and as the same aetherial waves give rise to the phenomena of
light, then it ought to follow that light has equally a repulsive power
on the planets with which it comes into contact. If that can be proved,
and I submit that it can, then from the phenomena of light, we learn
that there is a force, or rather a motion, ever proceeding from the sun
in straight lines, as shown in the previous Art., which decreases in
power or intensity inversely as the square of the distance. So that not
only is the sun the centre of an attractive Force, the Centripetal
force, or the attractive Force of Gravity, which proceeds in straight
lines through space, decreasing in intensity inversely as the square of
the distance, but at the same time it is also the centre of a motion,
that is, the aetherial wave motion of light, which takes exactly the
same path as Gravitation Attraction, and which is subject to exactly the
same laws. Unlike Gravitation Attraction, however, its power and motion
is ever directed away from the central body, the sun; and if such motion
exerts any power on any planet with which it comes into contact, that
power or motion can only be a repulsive motion in the same way that heat
is a repulsive motion. Assuming, therefore, that light, through the
medium of the aetherial waves, does exert this repulsive motion, then,
according to the law of inverse squares, it can be seen that if a
planet's distance be doubled, the repelling power of the aetherial light
waves would be decreased 1/4, while at the same time the attractive
power of the centripetal force would be decreased 1/4 also. If, on the
other hand, the planet's distance from the sun be reduced to 1/2 of its
former distance, then the repelling power of the aetherial light waves
would be increased four times, but contemporaneously with the increase
there would be an increase in the attractive power of gravity, which
would exactly counterbalance the increased repelling power of the light
waves. So that in assuming that there is this repelling power in the
light waves, there are thus two forces in existence in the solar system
(which is a type of all other systems), or rather two motions, as all
forces resolve themselves into motions of some kind, one motion ever
tending from the central body, that is, the motion of the aetherial
light waves, and the other tending to the central body, that is, the
force of gravity, which we shall see later on is also a motion of the
Aether, whose influence is ever towards the central body, be it a sun,
star, or planet. These two motions, therefore, are subject to the same
law, viz. that their power or intensity is not only directed in straight
lines from the central body, but their intensity is regulated exactly by
the same law of inverse squares. If the repelling motion be doubled,
then the attractive motion or power would be doubled also; if the
repelling motion be quadrupled or halved, then the attractive force of
gravity would be quadrupled or halved in the same way, the two forces
being exactly increased or decreased in the same ratio according to the
law of inverse squares.
ART. 76. _Velocity of Light._--The transmission of light is not
instantaneous, as it requires time for its propagation through space,
from the luminous body which gives rise to all light, as the sun for
example, until it reaches the body which it lights up. The velocity of
the light waves, however, is so great, that it is almost impossible to
give any comparative idea of their rate of transmission. The velocity of
the light waves was first established by Roemer, a Danish astronomer, in
1675. He ascertained the velocity of light by observations made on the
satellites of Jupiter. His methods of reasoning can easily be understood
by reference to the following diagram.
[Illustration: Fig: 8.]
Let _S_ represent the sun, and _A_ and _B_ the orbit of the earth round
the sun; _C_ _E_ _D_ part of Jupiter's orbit round the sun; while _D_
_E_ _F_ represents the orbit of Jupiter's satellite. When the earth,
Jupiter, and the satellite are in a straight line with each other, the
satellite suffers an eclipse through passing into the shadow thrown by
Jupiter. Now Roemer found that there was a difference in the time of the
eclipse when the earth was at _B_, that is, when it was nearest to
Jupiter, and when it was at _A_, which is that part of the earth's orbit
furthest away from Jupiter. That difference was accounted for by the
fact, that when the earth was at _A_ the light had to travel further
from Jupiter than when the earth was nearest to Jupiter, that is at
point _B_. Thus, when the earth was nearest to Jupiter, the light had a
shorter journey to travel than when it was furthest from Jupiter. The
difference, he found, was about 16 minutes, and he reasoned that this
difference was caused by the light having to cross the earth's orbit
from _B_ to _A_, in its longer journey, than when it only had to reach
the earth at _B_. The mean distance of the earth from the sun, that is,
the radius of the earth's orbit, is about 92-1/2 million miles, so that
the diameter of the earth's orbit is about 185,000,000 miles, and if it
takes about 16 minutes for light to traverse this distance, we find that
light has a velocity, according to Roemer, of 192,500 miles per second.
The result, however, arrived at by Roemer was not generally accepted at
that time, and it was not till 1728 that Bradley discovered what is
known as the Aberration of Light, and from that discovery proved that
light was not transmitted instantaneously through space, but that it was
transmitted with finite velocity; and that that velocity corresponded
fairly well with the velocity given by Roemer. Bradley, in his
astronomical observations, noticed that some of the fixed stars, so
called, did not appear to be really fixed, but that they described small
circles in the heavens each year. This fact greatly perplexed him, until
at last he hit upon the true solution by taking into account the motion
of the earth in its orbit, together with the fact that light had a
finite velocity. This result showed that the light from the stars
travelled with the same velocity as that which travelled from Jupiter's
satellites. The Aberration of Light, as his discovery was termed, may be
illustrated in the following way--Suppose that you are standing still,
and that it is raining, the rain descending vertically on the umbrella
that you hold up to cover you. As soon as you begin to walk, the
rain-drops will apparently begin to slant, and if the walk is changed
into a run, the greater apparently will be the slanting direction that
the rain-drops take. In the same way, the rays of light from a star
would fall vertically upon the earth if it were motionless, but as the
earth is moving through space with varying velocity, it gives to the
rays of light a slanting direction. By calculating the speed of the
earth, and ascertaining the exact slanting direction of the rays, the
velocity of light may be ascertained. This Bradley did, and showed that
it coincided almost with the result arrived at by Roemer. Various other
means have been adapted to test the results arrived at by these two
astronomers. Fizeau, in 1849, was able to measure the velocity of light
by using, not planetary or stellar distances, but by simply using
distances in the city of Paris; while Foucault, in 1860, devised a
method of measuring the velocity of light in air or any other medium.
The results arrived at by these men leave no doubt as to the exact speed
of light, which may now be reckoned to have a velocity of 186,000 miles,
or 300,000,000 metres per second. Notwithstanding this great speed at
which light travels, the nearest stars are so far off that their light
takes about 3-1/2 years to reach the earth, while scientists tell us
that some of the most distant stars are so remote, that their light
takes thousands of years to reach our earth, travelling at the rate of
186,000 miles per second. From considerations like these we get a dim
conception of the almost illimitable extent of the universe. Now let us
try to understand what this rate of motion really means. We have to
remember that light is caused by wave motions in the Aether, so that we
have here a wave motion which is travelling through the Aether at the
enormous rate already quoted. Light takes about 8-1/2 minutes to travel
from the sun to the earth, a distance of 92,000,000 miles. Our fastest
trains do not travel 80 miles an hour, and if a train left the sun and
continued its journey through space at that rate, it would take over 130
years before it reached our earth, while the light would perform the
journey in 8-1/2 minutes. We have some idea of the velocity of a train
travelling at 80 miles an hour; what, however, must be the velocity of a
wave motion which travels 22,500 times as fast? In Art. 56 we have seen
that all energy is the energy of motion, and therefore wherever we get
motion of any kind or sort, there we must have energy accompanying it,
or the power to do work. We have here, then, a source of energy in the
aetherial waves known as light waves, with their enormous velocity which
is almost inconceivable and illimitable. What must be the energy which
exists in space due to the wave motion of the Aether? We have to
remember on this point that we are no longer dealing with a frictionless
medium, but that we are dealing with matter, only in a far more rarefied
and far more elastic form than ordinary matter, but nevertheless matter
just as air is considered matter, and, being matter, its very motion
imparts to the light waves a power and a force which make them capable
of doing work. The kind of work done will be considered later on, when
we deal with the dynamical value of light. That we do not feel the power
and energy of the light waves is due to the well-known fact that their
power is broken by the activity of the atmospheric particles, each of
which, in their myriads, is ever moving with great velocity, and
therefore bombard the light waves, as they endeavour to strike the
earth. Thus the aetherial light waves are broken up and shattered, and
fall to the earth not with their full energy or power, but in a blended
form, or with that reflected energy which we call light. If they were to
come unbroken and unchecked upon us, and on the earth, in the same way
that they apparently do upon our satellite the moon, we doubtless should
experience very different effects of their energy and power due to their
enormous velocity.
ART. 77. _Dynamical Value of Light._--We have already learned (Art. 68)
that heat possesses a dynamical value, such value being measured by
Joule, and its equivalent in foot-pounds being exactly ascertained. We
have further seen (Art. 69, on the identity of light and heat), that the
same aetherial waves which produce heat are also concerned in the
production of light. If, therefore, the aetherial waves which give rise
to heat possess a dynamical action and equivalent, it follows that light
must also possess a dynamical action and equivalent, and such action
should be capable of being expressed in terms of foot-pounds. Clerk
Maxwell has recorded the exact dynamical equivalent of light. On this
matter he writes:[15] "If in strong sunlight the energy of light which
falls upon a square foot is 83.4 foot-pounds per second, the mean energy
of one cubic foot of sunlight is about .0,000,000,882 of a foot-pound,
and the mean pressure on a square foot is .0,000,000,882 of a pound
weight." We have here then the exact dynamical equivalent, according to
Maxwell, of a cubic foot of sunlight near the earth's surface, and of
the pressure exerted by light on a body with which it comes into
contact.
Again, Lord Kelvin[16] has measured the exact dynamical equivalent of a
cubic mile of sunlight, both near the surface of the sun and then near
the surface of the earth, and in a note adds that the relation of the
two values is as 46,000 to 1. So that if the dynamical value of a cubic
mile of sunlight near the earth's surface be represented by unity, then
the value of a cubic mile of sunlight near the sun's surface would be
46,000 times greater, while he further adds that it would take 4140
horse-power every minute, as the amount of work required to generate the
energy existing in a cubic kilometre of light near the sun, a kilometre
being equal to about 1093 yards.
Professor Challis[17] stated in 1872 that "Light is to be ranked with
the physical forces, and its dynamical action is equally to be ascribed
to the pressure of the Aether." Now I want to put this question to the
reader: If light possesses this dynamical action, that is, if it
possesses a motive or driving power, what must be the exact effect of
the dynamical action of the light waves from the sun upon all the
planets and meteors that revolve round it? We know that the sun is
324,000 times the mass of our earth, and that it has a diameter of about
856,000 miles and a circumference of over two million and a half miles.
What, therefore, must be the energy of the aetherial light waves that it
speeds on their way through space on every side? Stokes,[18] in regard
to the mechanical energy of Light, states that "the amount of energy
poured forth into space corresponds in round numbers to 12,000
horse-power per square foot," and that every square foot of the sun's
surface supplies energy at the above rate. The number of feet in the
sun's surface can be approximately determined. Roughly, there are
2,284,000,000 square miles of surface on the sun's huge form, and there
are 27,878,400 square feet in a mile. By multiplying these two numbers
we can ascertain the exact number of square feet on the surface of the
sun. If, therefore, every square foot possesses a mechanical value equal
to 12,000 horse-power, what must be the mechanical equivalent of the
sun's radiation of light that it pours forth into space?
I want to call the attention of the reader to another fact, and that is,
that light always proceeds in straight lines from the sun (Art. 76), and
therefore if there be any mechanical action in light at all, that action
must be one which is always directed from the sun in straight lines. Now
experience universally teaches us, that if a body is pushed, and pushed
with such a force as has been indicated, then that body not only moves,
but moves in the direction that the supposed horses would push. I have
already shown (Art. 76) that the path of light is that of a straight
line corresponding to the path of the attractive force of gravity;
therefore these horses must ever push in a direction _from_ the sun
along the same path that the sun's attractive power takes. In other
words, the mechanical action of these supposed horses will be a
repulsive one, that repulsion being due to the dynamical action of the
light waves upon the body that they come into contact with. If this is
correct, then not only is heat a repulsive motion, as stated in Art. 63,
but light is equally the possessor of a repulsive motion, because its
action is ever directed from the sun. We might continue to follow the
supposed horses as they continued their course through space, and we
should find that their energy decreased inversely as the square of the
distance, partly because the further they proceeded into space the
larger the area would be they would have to cover, and therefore their
energy would be decreased proportionately.
Professor Stokes, in the same work[19] already referred to, in
continuation of the same idea, states: "At the distance of the earth the
energy received would correspond to about one horse-power for every
square of 5 feet, on that side of the earth's surface facing the sun,
supposing the rays to fall perpendicularly." That being so, we can
exactly calculate in horse-power the energy received from light on that
side of the earth facing the sun, at its distance of 92,000,000 miles.
The area of the earth's surface is, roughly, 200,000,000 × 5280 square
feet, and if the energy received is equal to one horse-power for every 5
square feet, then the amount of energy received by the earth on that
side facing the sun would be equal to 200,000,000 × 5280 × 1/2 × 1/5
horse-power. This power, it must be remembered, is ever directed _away
from_ the sun, and upon that side of the planet that faces the solar
orb. So that we have virtually a repulsive force ever directed against
the earth, estimated by Professor Stokes to be equal to the estimated
horse-power.
This assumption of the repulsive power of light brings the phenomena of
light into harmony with that of heat, because we have already seen (Art.
63) that heat is essentially a repulsive motion, as indicated by Davy,
Rumford and others; and, as heat and light both have a common origin,
then light should possess a repulsive power also.
As further proof of this statement, let me again quote from Clerk
Maxwell. In the quotation already given in this Art. we have seen that
the pressure of sunlight on a square foot is equal to 83.4 lb. He adds
the following words to those already quoted: "A flat body exposed to
sunlight would experience this pressure on its illuminated side only,
and would therefore be repelled from the side on which the light falls."
Now if more conclusive proof of the correctness of the argument I am
advancing were required, I do not think it could be given from any
greater authority than that just quoted. Coming from the pen of one of
the most brilliant scientists that the past century has known, I venture
to think the opinion will be received with that due weight which it
demands.
This statement of Clerk Maxwell's has received, however, definite and
experimental proof from Professor Lebedew of Moscow University, and by
Nichols and Hull of America. The former has given, in the _Annalen der
Physik_ for November 1901, the results of his experiments in relation to
the pressure of light. The following are the results: He proved, 1st,
that the incident beam of light exerts pressure both upon an absorbing
and a reflecting body; 2nd, that the pressure of light is proportionate
to the amount of incident energy, and is independent of the colour of
light; 3rd, that the pressure of light corresponds with the forces of
radiation as calculated by Maxwell.
About the same time, Nichols and Hull of America were engaged upon
experiments relating to the pressure of light waves, and their results
were published in the November _Physical Review_, 1901. Thus, from two
separate and independent sources, Maxwell's equations as to the pressure
which light waves exert upon any body on which they fall received
definite experimental confirmation.
The repulsive power of the light waves receives further confirmation from
that theory known as the electro-magnetic theory, which supposes light to
be nothing more or less than an electro-magnetic phenomenon; that is to
say, it is directly or indirectly due to the action of electric currents.
As already indicated, Lorentz was of the opinion that the light waves
were themselves electric currents, and whether this is the actual fact
or not, certainly it is true that the electro-magnetic theory of light
is no mere fable or myth, but that it ranks as one of the most advanced
and correct hypotheses in regard to light that has ever been given to
the world. According to that theory, which we shall look at
subsequently, we find that the aetherial medium is not only the medium
for the light waves, but that it is also the medium which conveys and
carries the electric currents through space, and even through all
matter. Further, from that theory we shall have good reasons for
assuming that the aetherial light waves are either themselves electric
currents, or closely identified with them, in the same way that the
light waves are identified with heat waves. If these facts should be
found to hold good relative to the identity of aetherial light waves and
the aetherial electric waves, then it can very readily be seen that such
a hypothesis gives added weight to the repulsive power of light. One of
the very commonest facts regarding electricity and its currents is, that
wherever we get electricity, we not only get attraction, but there is
always associated with that attractive force a repulsive force, which is
equal in amount to the attractive force. So that if, wherever we get
electric currents, we find associated with those currents a repulsive
force, then, in view of the electro-magnetic theory of light, it should
also follow that on that hypothesis we ought also to find a repulsive
power in light. From the dynamical aspect of light on the bases of facts
given to the world by such men as Professor Stokes, Clerk Maxwell, Lord
Kelvin, and Professor Lebedew, we are compelled, therefore, to come to
the conclusion that light does possess such a repulsive force, such
force being due to the dynamical action of the aetherial light waves.
Thus we learn from the dynamical action of light, that not only is the
sun the centre of an attractive force, but that it is equally the centre
of a repellent or repulsive power or motion; which repulsive power
always follows the path of the radius vector, and diminishes with an
intensity which is inversely as the square of the distance. What we have
to ask ourselves therefore is, whether the repulsive power of light is
the Centrifugal Force that we are trying to discover? In Art. 24 we
found out what were the necessary characteristics of the Centrifugal
Force, which is to form the companion law to the attractive law of
gravity, or the Centripetal Force. We there saw that this centrifugal
law must be universal in character; that it must coincide with the path
of the centripetal force; that it must also be subject to the same law
of intensity, viz. the law of inverse squares; and further, that the
force must be proportional to the product of the two masses concerned.
We find in the repulsive power of light three at any rate of these
conditions fulfilled. Light is universal because Aether is universal.
It is always subject to the law of inverse squares, and what is more,
its repelling power coincides exactly with the path which the
centripetal force takes, that is, the radius vector. We have not,
however, discovered that light fulfils the remaining necessary
condition, which is, that the repelling powers of light emitted by any
two bodies are equal to the product of their masses. So that until this
is done, it cannot be said that the aetherial light waves form the
centrifugal force or motion from a central body that we are seeking for.
But while that may be true, yet if light be not the centrifugal motion,
it certainly indicates in what direction we are to look for that force,
and that is to the Aether, whose periodic waves give rise to the
phenomena of light. For, after all, light is due to aetherial wave
motion, and, therefore, while light from certain standpoints may be
conceived to be the cause of other phenomena, yet primarily the real
cause of all phenomena which are due to light are due to the aetherial
waves which themselves give rise to the phenomena of light. Thus light
acts as a guide-post to us, pointing out the direction we should take in
order to find out the real centrifugal force or motion, and as plainly
as it possibly can, it indicates to us that the true solution of our
centrifugal motion that we are seeking for is to be found, and alone
found, in that universal aetherial medium which, by its vibrations and
wave motions, gives rise to that which we term Light. In conclusion of
this point, it may be pointed out that Professor Challis[20] also took
this view of light, as he distinctly states that "Light is to be ranked
with the physical forces, and its dynamical action is equally to be
ascribed to the pressure of the Aether," and then proceeded to show how
repulsion could be exerted on atoms by the periodic wave motion of the
Aether.
[Footnote 15: _Magnetism and Electricity._]
[Footnote 16: _Phil. Mag._, 1902.]
[Footnote 17: _Ibid._, 1872.]
[Footnote 18: _Burnet Lectures._]
[Footnote 19: _Burnet Lectures._]
[Footnote 20: _Phil. Mag._, 1872.]
ART. 78. _The Electro-Magnetic Theory of Light._--We have seen (Art.
71) that light is due to a periodic wave motion of the Aether, and we
have previously seen that heat is also due to a periodic wave motion of
the Aether. Thus in the phenomena of light and heat, Aether is the
medium in which the energy of light is stored, and by which it is
transmitted in its passage from a luminous body, as the sun, until it
comes into contact with a planet or satellite from which it is
reflected, thus giving rise to light and heat. When, however, we come to
deal with electro-magnetic phenomena, which are the results and effects
produced by electricity and magnetism, we find certain phenomena similar
to those that we find in relation to light and heat. Thus, when light is
emitted by a luminous body, a certain amount of energy is given out by
that body, and if such light is absorbed by another body, the latter
becomes heated, a clear proof that it has received energy or motion from
some outside source. From the time it left the luminous body till it
reached the lighted or absorbing body, it must have existed as energy,
that is, motion in the Aether. As we have already seen, Newton thought
that the transference of energy was accomplished by the actual
transference of certain small corpuscles or atoms given out by the
luminous body, which conveyed the energy of the one body to the other.
According to the wave theory of light, however, we find that the
transference of energy is accomplished by a wave motion in the Aether,
which is periodic both in time and space, by which wave motion the
energy is transferred from the luminous to the illuminated body. Now
every one is familiar with the effects of magnetism and electricity in
some form or other, and such familiarity teaches that various kinds of
work may be done by electricity. If an electric current be generated and
allowed to flow through any circuit, as the ramifications, for example,
of an electric-tram system, it can readily be seen that by the action of
the current large masses or bodies as trams may be moved. To generate
the current requires the expenditure of energy, and for the tram to be
moved requires the transmission of that energy from the generating
station till it reaches the body to be moved. By what means is such
energy transmitted? because if it disappears at one place and reappears
at another, it must have passed through a medium during the interval. It
has been demonstrated that the medium which conveys the current from
place to place is the Aether, so that as light is transmitted through
space by the Aether, in a similar manner electric currents are
transmitted through space also by the same medium. The discoverer of
this great truth was Clerk Maxwell, and it was from the consideration of
electro-magnetic phenomena that he was able to lay the foundation of
that theory known as the Electro-Magnetic Theory of Light. In paragraph
781 of his greatest work[21] he says: "In several parts of this treatise
an attempt has been made to explain electro-magnetic phenomena by means
of mechanical action from one body to another by means of a medium
occupying space between them. The undulatory theory of light also
assumes the existence of a medium. We have now to show that the
properties of the electro-magnetic medium are identical with those of
the luminiferous medium." He then points out that, "to fill all space
with a new medium, whenever any new phenomenon is to be explained, is by
no means philosophical"; and further adds, that "If it should be found
that the velocity of propagation of electro-magnetic disturbances is the
same as the velocity of light, and this not only in air, but in other
transparent media, we shall have strong reasons for believing that light
is an electro-magnetic phenomena." In the wave theory of light we have
seen (Art. 70), that two properties are necessary to any medium before
it is capable of transmitting wave motion of any kind. Those two
properties are elasticity and inertia. Water possesses these properties,
and so can transmit ocean waves; air also possesses these properties,
and so can transmit sound waves; and Aether, being matter, also
possesses these properties (Arts. 47 and 48), and is therefore capable
of transmitting light waves. The elasticity is essential in order for
the medium to store up energy, and also to enable it to resume its
original shape after deformation, while the inertia is necessary in
order that the medium may transmit the impulse, and oscillate to and fro
until the impulse received has been passed on. This elasticity and
inertia may be well illustrated by the bending of a lath or cane. If we
pull one end down, holding the other end quite still, we shall see that
the lath oscillates to and fro until gradually it comes to rest. The
elasticity of the lath allows it to be pulled out of its original
position, and also enables it to rebound, while its inertia causes it to
swing back again past its original position. Both combined together
cause it to swing backwards and forwards till its energy is used up. If
such a series of springs could be set in motion at equal intervals of
space and time, we should then have a good illustration of a wave
motion.
What analogy, may be asked, is there in electro-magnetic phenomena to
correspond to this elasticity and inertia of the Aether, so essential to
the propagation of light? Let us look at the familiar illustration of
charging a Leyden jar. In charging a Leyden jar with electricity a
certain amount of energy is spent, work is done, and the result is found
in the electrified state of the jar. That which has actually been
accomplished is the storing up of energy in the Aether around the jar.
This storing up of energy is analogous to pulling aside the lath, and is
the making use of the elasticity of the Aether, in order to produce a
tendency to recoil. When the jar is discharged, which is analogous to
letting go the lath, the Aether seeks to recover its former condition by
discharging the energy it received. In these operations the elasticity
of the Aether is called into play. After the jar is discharged, however,
the recoil of the Aether produces a current, and the inertia of the
current causes it to overshoot its original position, and for an instant
the charge of the jar is reversed. The current now flows backwards in
the same way that the lath returned back, and charges the jar as at
first. This discharging and recharging continue backward and forward, so
to speak, until all the energy which was originally given to the jar has
been expended, and it resumes its normal condition. In this experiment
the elasticity and inertia of the Aether have both been called into
play, so that we see in this electrical experiment a similar
illustration of the elasticity and inertia of the Aether, as manifested
in the undulatory or wave theory of light. The question now arises, what
are the corresponding properties as given by Maxwell in his
electro-magnetic theory? In Art. 782 he writes: "In the theory of
electricity and magnetism adopted in this treatise two forms of energy
are recognized--the electro-static and the electro-kinetic--and these
are supposed to have their seat, not merely in the electrified or
magnetized bodies, but in every part of the surrounding space where
electric or magnetic force is observed to act. Hence our theory agrees
with the undulatory theory in assuming the existence of a medium which
is capable of becoming a receptacle of two forms of energy." Faraday, in
his _Experimental Researches_, paragraph 3075, in referring to the
character of magnetic phenomena external to the magnet, writes: "I am
more inclined to the notion that in the transmission of force there is
such an action external to the magnet, than that the effects are merely
attraction and repulsion at a distance. Such a function may be a
function of the Aether if it should have other uses than simply the
conveyance of radiations" (light and heat). From this extract we learn
that Faraday was also of the opinion that the Aether around a magnet or
any electrified body was directly concerned in the propagation of the
electric and magnetic forces, these forces according to Maxwell being of
two kinds. From the illustration of the charging and discharging of the
Leyden jar, we learn that aetherial electrical waves can be produced by
electric means, and from the alternate charging and recharging of the
jar we learn that these aetherial waves travel to and from the jar with
a periodic wave motion. Here, therefore, we have an aetherial wave
motion which is produced wholly by electricity, and yet which answers
our definition of a wave motion of light, in that it is periodic both
in time and space, and in that aetherial wave motion Maxwell states that
two forms of energy are called into play, which he calls Electro-Static
and Electro-Kinetic. These correspond respectively to the elasticity and
inertia in the older theory of the wave motion of light. It was upon
this basis that Maxwell built up his electro-magnetic theory. Even this
theory does not tell us what is the exact character or nature of the
periodic wave motion of the Aether. All it tells us is, that the
electro-magnetic wave motion of the Aether is the same in nature and
character as the wave motion which produces light and heat. Thus it
shows that light and electricity have a common origin, and proves that
light is nothing more or less than an electro-magnetic phenomenon.
Maxwell gave a number of proofs in support of his theory. He showed that
the velocity of the electro-magnetic waves was almost identical with the
velocity of light waves, his results being as follows--
VELOCITY OF ELECTRO-MAGNETIC VELOCITY OF LIGHT IN
WAVES. METRES PER SEC.
Weber 310,740,000 metres per sec. Fizeau 314,000,000
Maxwell 288,000,000 " " Foucault 298,360,000
Thompson 282,000,000 " "
From these figures it can readily be seen that the velocity of an
aetherial wave, generated by electric means, is approximately the same
as an aetherial wave generated by a luminous body. Thus one of the most
important results of Maxwell's theory was to show that electro-magnetic
disturbances produced in the Aether by an electrified or magnetic body
might be propagated through space with a velocity equal to that of
light.
It was left, however, for Professor Hertz to place the electro-magnetic
theory of light upon a sure and certain foundation. The results are to
be found in his work on _Electric Waves_, translated by Professor Jones,
1893.
In his paper on "The Velocity of Propagation of Electro-dynamic Action,"
he gave definite and experimental proof of the hitherto theoretical fact
that the velocity of the electric waves in air was exactly the same as
that of light, whereas he found that in wires the ratio was not the
same, being 4 to 7. This was afterwards found out to be an error by some
experiments made at Geneva, when it was discovered that the propagation
in wires was the same as in the air. Among his experiments Hertz
succeeded in producing very short electric waves of 30 centimetres in
length, that is, about one and one-fifth of an inch. According to
Maxwell's theory, such a wave ought to behave exactly as a beam of
ordinary light does. Hertz proved that this was the case, and published
his proofs in his paper on "Electric Radiation." In that paper he showed
how such electric radiation was propagated in straight lines, like
light, and that it could also be refracted and reflected. Thus he gave
to Maxwell's electro-magnetic theory experimental demonstration, and
placed it on a solid and immovable foundation. In summing up the results
of this theory, we learn, therefore, that Hertz has conclusively proved
that electric and magnetic effects are propagated through the Aether
which fills all space with the same velocity that light is propagated.
Further, he has conclusively proved the identity between light and
electricity, and shown that electric and light radiations are
essentially one and the same, and that they are both propagated by
periodic wave motions of the Aether. Further, he has proved that the
velocity of the propagation of light is the same as that of the
electro-magnetic waves, and that these waves obey all the laws that
govern light and heat. We have here, therefore, experimental proof of
the identity between electricity and light, and in Art. 69 we have also
proved the identity of light and heat, so that we have now experimental
proof that light, heat and electricity are all due to the periodic wave
motions and vibrations of the universal Aether, which not only fills all
space, but which surrounds every atom and every particle of matter
throughout the whole universe. Having established, therefore, the
identity of heat, light and electricity, and having proved that they are
all due to the periodic wave motions or vibrations of the universal
Aether, it must follow as a matter of necessity that wherever in
interplanetary or interstellar space we find light or heat waves we must
also find electricity. We have already seen that aetherial light waves
flood all space, both interplanetary and interstellar space, so that in
view of the identity of the aetherial light waves and aetherial electric
waves, it follows that the aetherial electric waves flood all space in
the same way, and at the same time. Wherever, therefore, we find the
light waves, there we find the electric waves also; and it will be
impossible to find the one without the other. Thus, throughout all
space, and indeed throughout the universe, light waves will not be found
apart from electric waves. They are as incapable of being dissociated as
are light and heat waves. Now we have already seen (Art. 64), so far as
the solar system is concerned, that the sun is the generator of all
light and heat, and that these light waves speed from the sun on every
side with a velocity of 186,000 miles per second. From the identity of
light and electric waves, therefore, given to us by the electro-magnetic
theory of light, it must follow that the sun is equally the source and
generator of the electric waves. Not only so, but as the light waves
flood all solar space, these electric waves, being identical with the
light waves, must flood the solar system also. Thus we learn from
Maxwell's theory as developed by Hertz, that not only is the sun the
generator of light and heat waves which are poured forth into space
continually with a velocity almost inconceivable, but at the same time
the sun is pouring forth into space electric waves which travel outwards
in spherical shells in the same way as light waves do, and with a
similar intensity, as we shall see in the next chapter. Now let me ask
the reader to ponder over the fact given to us by this electro-magnetic
theory in its relation to the solar system, and endeavour to find out
what such an application teaches us. Let it be remembered that we are
looking for a Centrifugal force or motion, that is, a motion from a
centre, which is to be the exact counterpart of the Centripetal force,
_i. e._ motion to a centre; and further, that the Centrifugal motion
must be a repulsive motion acting in the opposite way to the attractive
power of the Centripetal force, that is, the attractive power of
gravity. We have seen (Art. 77) that light possesses a repulsive power.
We have now only to prove that electricity or the aetherial electric
waves have a repulsive motion, which will be the easiest of all to
prove, and then we shall have proved beyond the possibility of
contradiction, the existence of that repulsive force referred to by
Herschel in Art. 24, which is to form the complementary and counterpart
of the attractive power of gravity. If it can be proved that electricity
does possess such a power, that is, a repulsive power, ever acting from
a centre, then in view of the identity of light, heat and electricity,
the correctness of the views we have advanced as to the repulsive power
of light and heat will be proved beyond the shadow of a doubt, otherwise
Maxwell's electro-magnetic theory of light is a fable and a myth, and
Hertz' experiments were never performed. Further, if all
electro-magnetic phenomena are due to the same aetherial medium which
gives rise by its wave motions to light, heat and electricity, then we
shall have discovered a medium which throughout the universe can by its
wave motions transmit and propagate both repulsions and attractions,
that is, the aetherial medium which is to be the physical cause of
Universal Gravitation. In order to further develop and establish this
point we will now consider the subject of Electricity as a Mode of
Motion.
[Footnote 21: _Mag. and Elec._]