Chapter VII: Light, a Mode of Motion (1)

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, CO_2, and so give rise to chemical decomposition, has been proved by Professor Tyndall and others, but I have never yet seen any record of any attempt to find out what these chemical waves are. There may have been such attempts made to discover their origin and character, but I have not seen any such record. I purpose, therefore, to offer an explanation as to the character and origin of these chemical or actinic waves, which I hope to prove by philosophical reasoning. We have already seen (Arts. 54 and 59) that both heat and light are convertible, or can be transformed into electricity, so that the same aetherial wave motion which can produce light can also produce heat, and that in its turn can produce electricity. Thus we learn that there is a very close identity between light, heat, and electricity; indeed it can be demonstrated that the same aetherial wave motion which produces electricity can produce the other two.