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.

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.

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?

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.

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, CO₂, 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,[13] 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.

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.

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:[14] “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[15] 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[16] 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,[17] 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[18] 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[19] 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.

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[20] 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--

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.