The ill.u.s.tration 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_?
[Ill.u.s.tration: Fig: 6.]
The reply is, by means of the particles, or atoms of air that exist between the two surfaces of the balloons, and that transmission would take the form of a wave propagated from particle to particle, so that we might put dots on the right side of _A_ to represent the atoms of air which transmit the wave from _A_ to _B_.
But the vibration which takes place in the surface of the envelope of the outer balloon is _across_ this line of propagation, because as the wave proceeds from _A_ to _B_, the elastic envelope expands and stretches always _across_ the line of propagation--that is, it stretches up and down, left and right, as it is expanded outwards, so that the vibration or oscillation of the particles always takes place in the surface of the elastic envelope across the line of propagation. Let us therefore apply the result of this simple experiment to our solar system and the Aether, and see if it can be made to explain the transverse vibration of light. Let _A_ represent the sun (Fig. 7) and _B_ an aetherial elastic envelope surrounding the sun. In this case we dispense with the bulb _C_, as the sun possesses within itself the power to generate heat, and so to produce the required expansion of the elastic aetherial envelopes _B_, _G_, _H_, etc.
[Ill.u.s.tration: Fig: 7.]
Instead, however, of having air particles between _A_ and _B_, we will put in their place our aetherial atoms which we have conceived according to Art. 44. These surround the sun, represented by _A_, forming elastic spherical sh.e.l.ls or envelopes. As the sun radiates its heat into s.p.a.ce, 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 sh.e.l.l 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 sh.e.l.l 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 s.p.a.ce 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. .h.i.therto 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 pa.s.sage and turns it back. We have ill.u.s.trations 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 pa.s.sing from one medium into another, it is turned from the straight path in which it was going before it entered the refracting medium. An ill.u.s.tration of the refraction of light is to be found in the case of the gla.s.s 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 a.s.sumes that reflection and refraction of light are dependent upon different degrees of density of the Aether a.s.sociated 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 a.s.sociation 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 a.s.sociation 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 acc.u.mulated 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 h.o.m.ogeneous 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 gla.s.s 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 pa.s.ses 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 a.s.sociation 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, a.s.suming 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 r.e.t.a.r.ded 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 a.s.serted 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 s.p.a.ce not being composed of atoms, while the electricity a.s.sociated 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 a.s.sociation with the Aether, then they must be of two kinds, positive and negative, as it is impossible to find positive electricity disa.s.sociated 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 pa.s.sed through a prism, it is broken up or decomposed into its const.i.tuent parts. This is called dispersion, and conclusively proves that the light from the sun is not a simple, but a compound colour. We have ill.u.s.trations 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 pa.s.ses 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 pa.s.sed 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 cla.s.s 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 ident.i.ty between light, heat, and electricity; indeed it can be demonstrated that the same aetherial wave motion which produces electricity can produce the other two.
Lorentz,[14] in an article on "The Ident.i.ty 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 a.n.a.logy 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 s.p.a.ce, must therefore be equally concerned in the propagation of electric displacements in the free Aether; as he states, it would be philosophically wrong to a.s.sume 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 a.s.sociated with electric waves in the same way that they are a.s.sociated 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 cla.s.ses 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 a.s.sert 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 pa.s.sing 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 s.p.a.ce with the velocity of light.
[Footnote 14: _Phil. Mag._, 1867.]
ART. 74. _Direction of Ray of Light._--In Art. 65 it was shown that the direction of a ray of heat was that of a straight line from the heated or luminous body from which the Aether waves proceeded. We have also seen in Art. 69 that the aetherial waves which give rise to the phenomena of heat are identical with those that give rise to light, so the direction of a ray of light must also be that of a straight line proceeding from the luminous body. A ray of light is a line perpendicular to the Aether waves which are propagated through s.p.a.ce 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 pa.s.s into the darkened room. The beam of light which pa.s.ses 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 pa.s.sing 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 pa.s.s 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.