Six Lectures on Light Part 8

With the wave-lengths once at our disposal, we follow the ether into the most complicated cases of interaction between it and ordinary matter, 'the theory is equal to them all. It makes not a single new physical hypothesis; but out of its original stock of principles it educes the counterparts of all that observation shows. It accounts for, explains, simplifies the most entangled cases; corrects known laws and facts; predicts and discloses unknown ones; becomes the guide of its former teacher Observation; and, enlightened by mechanical conceptions, acquires an insight which pierces through shape and colour to force and cause.'[18]

But, while I have thus endeavoured to ill.u.s.trate before you the power of the undulatory theory as a solver of all the difficulties of optics, do I therefore wish you to close your eyes to any evidence that may arise against it? By no means. You may urge, and justly urge, that a hundred years ago another theory was held by the most eminent men, and that, as the theory then held had to yield, the undulatory theory may have to yield also. This seems reasonable; but let us understand the precise value of the argument. In similar language a person in the time of Newton, or even in our time, might reason thus: Hipparchus and Ptolemy, and numbers of great men after them, believed that the earth was the centre of the solar system. But this deep-set theoretic notion had to give way, and the helio-centric theory may, in its turn, have to give way also. This is just as reasonable as the first argument. Wherein consists the strength of the present theory of gravitation? Solely in its competence to account for all the phenomena of the solar system. Wherein consists the strength of the theory of undulation? Solely in its competence to disentangle and explain phenomena a hundred-fold more complex than those of the solar system.

Accept if you will the scepticism of Mr. Mill[19] regarding the undulatory theory; but if your scepticism be philosophical, it will wrap the theory of gravitation in the same or in greater doubt.[20]

-- 11. _The Blue of the Sky_.

I am unwilling to quit these chromatic phenomena without referring to a source of colour which has often come before me of late in the blue of your skies at noon, and the deep crimson of your horizon after the set of sun. I will here summarize and extend what I have elsewhere said upon this subject. Proofs of the most cogent description could be adduced to show that the blue light of the firmament is reflected light. That light comes to us across the direction of the solar rays, and even against the direction of the solar rays; and this lateral and opposing rush of wave-motion can only be due to the rebound of the waves from the air itself, or from something suspended in the air. The solar light, moreover, is not scattered by the sky in the proportions which produce white. The sky is blue, which indicates an excess of the smaller waves. The blueness of the air has been given as a reason for the blueness of the sky; but then the question arises, How, if the air be blue, can the light of sunrise and sunset, which travels through vast distances of air, be yellow, orange, or even red? The pa.s.sage of the white solar light through a blue medium could by no possibility redden the light; the hypothesis of a blue atmosphere is therefore untenable. In fact, the agent, whatever it be, which sends us the light of the sky, exercises in so doing a dichroitic action. The light reflected is blue, the light transmitted is orange or red, A marked distinction is thus exhibited between reflection from the sky and that from an ordinary cloud, which exercises no such dichroitic action.

The cloud, in fact, takes no note of size on the part of the waves of ether, but reflects them all alike. Now the cause of this may be that the cloud-particles are so large in comparison with the size of the waves of ether as to scatter them all indifferently. A broad cliff reflects an Atlantic roller as easily as it reflects a ripple produced by a sea-bird's wing; and, in the presence of large reflecting surfaces, the existing differences of magnitude among the waves of ether may also disappear. But supposing the reflecting particles, instead of being very large, to be very small, in comparison with the size of the waves. Then, instead of the whole wave being fronted and in great part thrown back, a small portion only is shivered off by the obstacle. Suppose, then, such minute foreign particles to be diffused in our atmosphere. Waves of all sizes impinge upon them, and at every collision a portion of the impinging wave is struck off. All the waves of the spectrum, from the extreme red to the extreme violet, are thus acted upon; but in what proportions will they be scattered? Largeness is a thing of relation; and the smaller the wave, the greater is the relative size of any particle on which the wave impinges, and the greater also the relative reflection.

A small pebble, placed in the way of the ring-ripples produced by heavy rain-drops on a tranquil pond, will throw back a large fraction of each ripple incident upon it, while the fractional part of a larger wave thrown back by the same pebble might be infinitesimal. Now to preserve the solar light white, its const.i.tuent proportions must not be altered; but in the scattering of the light by these very small particles we see that the proportions _are_ altered. The smaller waves are in excess, and, as a consequence, in the scattered light blue will be the predominant colour. The other colours of the spectrum must, to some extent, be a.s.sociated with the blue: they are not absent, but deficient. We ought, in fact, to have them all, but in diminishing proportions, from the violet to the red.

We have thus reasoned our way to the conclusion, that were particles, small in comparison to the size of the ether waves, sown in our atmosphere, the light scattered by those particles would be exactly such as we observe in our azure skies. And, indeed, when this light is a.n.a.lyzed, all the colours of the spectrum are found in the proportions indicated by our conclusion.

By its successive collisions with the particles the white light is more and more robbed of its shorter waves; it therefore loses more and more of its due proportion of blue. The result may be antic.i.p.ated. The transmitted light, where moderate distances are involved, will appear yellowish. But as the sun sinks towards the horizon the atmospheric distance increases, and consequently the number of the scattering particles. They weaken in succession the violet, the indigo, the blue, and even disturb the proportions of green. The transmitted light under such circ.u.mstances must pa.s.s from yellow through orange to red. This also is exactly what we find in nature. Thus, while the reflected light gives us, at noon, the deep azure of the Alpine skies, the transmitted light gives us, at sunset, the warm crimson of the Alpine snows.

But can small particles be really proved to act in the manner indicated? No doubt of it. Each one of you can submit the question to an experimental test. Water will not dissolve resin, but spirit will; and when spirit which holds resin in solution is dropped into water, the resin immediately separates in solid particles, which render the water milky. The coa.r.s.eness of this precipitate depends on the quant.i.ty of the dissolved resin. Professor Brucke has given us the proportions which produce particles particularly suited to our present purpose. One gramme of clean mastic is dissolved in eighty-seven grammes of absolute alcohol, and the transparent solution is allowed to drop into a beaker containing clear water briskly stirred. An exceedingly fine precipitate is thus formed, which declares its presence by its action upon light. Placing a dark surface behind the beaker, and permitting the light to fall into it from the top or front, the medium is seen to be of a very fair sky-blue. A trace of soap in water gives it a tint of blue. London milk makes an approximation to the same colour, through the operation of the same cause: and Helmholtz has irreverently disclosed the fact that a blue eye is simply a turbid medium.

-- 12. _Artificial Sky_.

But we have it in our power to imitate far more closely the natural conditions of this problem. We can generate in air artificial skies, and prove their perfect ident.i.ty with the natural one, as regards the exhibition of a number of wholly unexpected phenomena. It has been recently shown in a great number of instances by myself that waves of ether issuing from a strong source, such as the sun or the electric light, are competent to shake asunder the atoms of gaseous molecules.

The apparatus used to ill.u.s.trate this consists of a gla.s.s tube about a yard in length, and from 2 to 3 inches internal diameter. The gas or vapour to be examined is introduced into this tube, and upon it the condensed beam of the electric lamp is permitted to act. The vapour is so chosen that one, at least, of its products of decomposition, as soon as it is formed, shall be _precipitated_ to a kind of cloud. By graduating the quant.i.ty of the vapour, this precipitation may be rendered of any degree of fineness, forming particles distinguishable by the naked eye, or particles which are probably far beyond the reach of our highest microscopic powers. I have no reason to doubt that particles may be thus obtained whose diameters const.i.tute but a very small fraction of the length of a wave of violet light.

Now, in all such cases when suitable vapours are employed in a sufficiently attenuated state, no matter what the vapour may be, the visible action commences with the formation of a _blue cloud_. Let me guard myself at the outset against all misconception as to the use of this term. The blue cloud here referred to is totally invisible in ordinary daylight. To be seen, it requires to be surrounded by darkness, _it only_ being illuminated by a powerful beam of light.

This cloud differs in many important particulars from the finest ordinary clouds, and might justly have a.s.signed to it an intermediate position between these clouds and true cloudless vapour.

It is possible to make the particles of this _actinic cloud_ grow from an infinitesimal and altogether ultra-microscopic size to particles of sensible magnitude; and by means of these in a certain stage of their growth, we produce a blue which rivals, if it does not transcend, that of the deepest and purest Italian sky. Introducing into our tube a quant.i.ty of mixed air and nitrite of butyl vapour sufficient to depress the mercurial column of an air-pump one-twentieth of an inch, adding a quant.i.ty of air and hydrochloric acid sufficient to depress the mercury half an inch further, and sending through this compound and highly attenuated atmosphere the beam of the electric light, within the tube arises gradually a splendid azure, which strengthens for a time, reaches a maximum of depth and purity, and then, as the particles grow larger, pa.s.ses into whitish blue. This experiment is representative, and it ill.u.s.trates a general principle. Various other colourless substances of the most diverse properties, optical and chemical, might be employed for this experiment. The _incipient cloud_, in every case, would exhibit this superb blue; thus proving to demonstration that particles of infinitesimal size, without any colour of their own, and irrespective of those optical properties exhibited by the substance in a ma.s.sive state, are competent to produce the blue colour of the sky.

-- 13. _Polarization of Skylight_.

But there is another subject connected with our firmament, of a more subtle and recondite character than even its colour. I mean that 'mysterious and beautiful phenomenon,' as Sir John Herschel calls it, the polarization of the light of the sky. Looking at various points of the blue firmament through a Nicol prism, and turning the prism round its axis, we soon notice variations of brightness. In certain positions of the prism, and from certain points of the firmament, the light appears to be wholly transmitted, while it is only necessary to turn the prism round its axis through an angle of ninety degrees to materially diminish the intensity of the light. Experiments of this kind prove that the blue light sent to us by the firmament is polarized, and on close scrutiny it is also found that the direction of most perfect polarization is perpendicular to the solar rays. Were the heavenly azure like the ordinary light of the sun, the turning of the prism would have no effect upon it; it would be transmitted equally during the entire rotation of the prism. The light of the sky may be in great part quenched, because it is in great part polarized.

The same phenomenon is exhibited in perfection by our actinic clouds, the only condition necessary to its production being the smallness of the particles. In all cases, and with all substances, the cloud formed at the commencement, when the precipitated particles are sufficiently fine, is _blue_. In all cases, moreover, this fine blue cloud polarizes _perfectly_ the beam which illuminates it, the direction of polarization enclosing an angle of 90 with the axis of the illuminating beam.

It is exceedingly interesting to observe both the growth and the decay of this polarization. For ten or fifteen minutes after its first appearance, the light from a vividly illuminated incipient cloud, looked at horizontally, is absolutely quenched by a Nicol prism with its longer diagonal vertical. But as the sky-blue is gradually rendered impure by the introduction of particles of too large a size, in other words, as real clouds begin to be formed, the polarization begins to deteriorate, a portion of the light pa.s.sing through the prism in all its positions, as it does in the case of skylight. It is worthy of note that for some time after the cessation of perfect polarization the _residual_ light which pa.s.ses, when the Nicol is in its position of minimum transmission, is of a gorgeous blue, the whiter light of the cloud being extinguished. When the cloud-texture has become sufficiently coa.r.s.e to approximate to that of ordinary clouds, the rotation of the Nicol ceases to have any sensible effect on the light discharged at right angles to the beam.

The perfection of the polarization in a direction perpendicular to the illuminating beam may be also ill.u.s.trated by the following experiment, which has been executed with many vapours. A Nicol prism large enough to embrace the entire beam of the electric lamp was placed between the lamp and the experimental tube. Sending the beam polarized by the Nicol through the tube, I placed myself in front of it, the eyes being on a level with its axis, my a.s.sistant occupying a similar position behind the tube. The short diagonal of the large Nicol was in the first instance vertical, the plane of vibration of the emergent beam being therefore also vertical. As the light continued to act, a superb blue cloud visible to both my a.s.sistant and myself was slowly formed.

But this cloud, so deep and rich when looked at from the positions mentioned, utterly disappeared when looked at vertically downwards, or vertically upwards. Reflection from the cloud was not possible in these directions. When the large Nicol was slowly turned round its axis, the eye of the observer being on the level of the beam, and the line of vision perpendicular to it, entire extinction of the light emitted horizontally occurred when the longer diagonal of the large Nicol was vertical. But a vivid blue cloud was seen when looked at downwards or upwards. This truly fine experiment, which I should certainly have made without suggestion, was, as a matter of fact, first definitely suggested by a remark addressed to me in a letter by Professor Stokes.

All the phenomena of colour and of polarization observable in the case of skylight are manifested by those actinic clouds; and they exhibit additional phenomena which it would be neither convenient to pursue, nor perhaps possible to detect, in the actual firmament. They enable us, for example, to follow the polarization from its first appearance on the barely visible blue to its final extinction in the coa.r.s.er cloud. These changes, as far as it is now necessary to refer to them, may be thus summed up:--

1. The actinic cloud, as long as it continues blue, discharges polarized light in all directions, but the direction of maximum polarization, like that of skylight, is at right angles to the direction of the illuminating beam.

2. As long as the cloud remains distinctly blue, the light discharged from it at right angles to the illuminating beam is _perfectly_ polarized. It may be utterly quenched by a Nicol prism, the cloud from which it issues being caused to disappear. Any deviation from the perpendicular enables a portion of the light to get through the prism.

3. The direction of vibration of the polarized light is at right angles to the illuminating beam. Hence a plate of tourmaline, with its axis parallel to the beam, stops the light, and with the axis perpendicular to the beam transmits the light.

4. A plate of selenite placed between the Nicol and the actinic cloud shows the colours of polarized light; in fact, the cloud itself plays the part of a polarizing Nicol.

5. The particles of the blue cloud are immeasurably small, but they increase gradually in size, and at a certain period of their growth cease to discharge perfectly polarized light. For some time afterwards the light that reaches the eye, through the Nicol in its position of least transmission, is of a magnificent blue, far exceeding in depth and purity that of the purest sky; thus the waves that first feel the influence of size, at both limits of the polarization, are the shortest waves of the spectrum. These are the first to accept polarization, and they are the first to escape from it.

LECTURE V.

RANGE OF VISION NOT COMMENSURATE WITH RANGE OF RADIATION THE ULTRA-VIOLET BAYS FLUORESCENCE THE RENDERING OF INVISIBLE RAYS VISIBLE VISION NOT THE ONLY SENSE APPEALED TO BY THE SOLAR AND ELECTRIC BEAM HEAT OF BEAM COMBUSTION BY TOTAL BEAM AT THE FOCI OF MIRRORS AND LENSES COMBUSTION THROUGH ICE-LENS IGNITION OF DIAMOND SEARCH FOR THE RAYS HERE EFFECTIVE SIR WILLIAM HERSCHEL'S DISCOVERY OF DARK SOLAR RAYS INVISIBLE RAYS THE BASIS OF THE VISIBLE DETACHMENT BY A RAY-FILTER OF THE INVISIBLE RAYS FROM THE VISIBLE COMBUSTION AT DARK FOCI CONVERSION OF HEAT-RAYS INTO LIGHT-RAYS CALORESCENCE PART PLAYED IN NATURE BY DARK RAYS IDENt.i.tY OF LIGHT AND RADIANT HEAT INVISIBLE IMAGES REFLECTION, REFRACTION, PLANE POLARIZATION, DEPOLARIZATION, CIRCULAR POLARIZATION, DOUBLE REFRACTION, AND MAGNETIZATION OF RADIANT HEAT.

-- 1. _Range of Vision and of Radiation_.

The first question that we have to consider to-night is this: Is the eye, as an organ of vision, commensurate with the whole range of solar radiation--is it capable of receiving visual impressions from all the rays emitted by the sun? The answer is negative. If we allowed ourselves to accept for a moment that notion of gradual growth, amelioration, and ascension, implied by the term _evolution_, we might fairly conclude that there are stores of visual impressions awaiting man, far greater than those now in his possession. Ritter discovered in 1801 that beyond the extreme violet of the spectrum there is a vast efflux of rays which are totally useless as regards our present powers of vision. These ultra-violet waves, however, though incompetent to awaken the optic nerve, can shake asunder the molecules of certain compound substances on which they impinge, thus producing chemical decomposition.

But though the blue, violet, and ultra-violet rays can act thus upon certain substances, the fact is hardly sufficient to ent.i.tle them to the name of 'chemical rays,' which is usually applied to distinguish them from the other const.i.tuents of the spectrum. As regards their action upon the salts of silver, and many other substances, they may perhaps merit this t.i.tle; but in the case of the grandest example of the chemical action of light--the decomposition of carbonic acid in the leaves of plants, with which my eminent friend Dr. Draper (now no more) has so indissolubly a.s.sociated his name--the yellow rays are found to be the most active.

There are substances, however, on which the violet and ultra-violet waves exert a special decomposing power; and, by permitting the invisible spectrum to fall upon surfaces prepared with such substances, we reveal both the existence and the extent of the ultraviolet spectrum.

-- 2. _Ultra-violet Rays: Fluorescence_.

The method of exhibiting the action of the ultraviolet rays by their chemical action has been long known; indeed, Thomas Young photographed the ultra-violet rings of Newton. We have now to demonstrate their presence in another way. As a general rule, bodies either transmit light or absorb it; but there is a third case in which the light falling upon the body is neither transmitted nor absorbed, but converted into light of another kind. Professor Stokes, the occupant of the chair of Newton in the University of Cambridge, has demonstrated this change of one kind of light into another, and has pushed his experiments so far as to render the invisible rays visible.

A large number of substances examined by Stokes, when excited by the invisible ultra-violet waves, have been proved to emit light. You know the rate of vibration corresponding to the extreme violet of the spectrum; you are aware that to produce the impression of this colour, the retina is struck 789 millions of millions of times in a second. At this point, the retina ceases to be useful as an organ of vision; for, though struck by waves of more rapid recurrence, they are incompetent to awaken the sensation of light. But when such non-visual waves are caused to impinge upon the molecules of certain substances--on those of sulphate of quinine, for example--they compel those molecules, or their const.i.tuent atoms, to vibrate; and the peculiarity is, that the vibrations thus set up are _of slower period_ than those of the exciting waves. By this lowering of the rate of vibration through the intermediation of the sulphate of quinine, the invisible rays are brought within the range of vision. We shall subsequently have abundant opportunity for learning that transparency to the visible by no means involves transparency to the invisible rays. Our bisulphide of carbon, for example, which, employed in prisms, is so eminently suitable for experiments on the visual rays, is by no means so suitable for these ultra-violet rays. Flint gla.s.s is better, and rock crystal is better than flint gla.s.s. A gla.s.s prism, however, will suit our present purpose.

Casting by means of such a prism a spectrum, not upon the white surface of our screen, but upon a sheet of paper which has been wetted with a saturated solution of the sulphate of quinine and afterwards dried, an obvious extension of the spectrum is revealed. We have, in the first instance, a portion of the violet rendered whiter and more brilliant; but, besides this, we have the gleaming of the colour where, in the case of unprepared paper, nothing is seen. Other substances produce a similar effect. A substance, for example, recently discovered by President Morton, and named by him _Thallene_, produces a very striking elongation of the spectrum, the new light generated being of peculiar brilliancy.

Fluor spar, and some other substances, when raised to a temperature still under redness, emit light. During the ages which have elapsed since their formation, this capacity of shaking the ether into visual tremors appears to have been enjoyed by these substances. Light has been potential within them all this time; and, as well explained by Draper, the heat, though not itself of visual intensity, can unlock the molecules so as to enable them to exert their long-latent power of vibration. This deportment of fluor spar determined Stokes in his choice of a name for his great discovery: he called this rendering visible of the ultra-violet rays _Fluorescence_.

By means of a deeply coloured violet gla.s.s, we cut off almost the whole of the light of our electric beam; but this gla.s.s is peculiarly transparent to the violet and ultra-violet rays. The violet beam now crosses a large jar filled with water, into which I pour a solution of sulphate of quinine. Clouds, to all appearance opaque, instantly tumble downwards. Fragments of horse-chestnut bark thrown upon the water also send down beautiful cloud-like strife. But these are not clouds: there is nothing precipitated here: the observed action is an action of _molecules_, not of _particles_. The medium before you is not a turbid medium, for when you look through it at a luminous surface it is perfectly clear.

If we paint upon a piece of paper a flower or a bouquet with the sulphate of quinine, and expose it to the full beam, scarcely anything is seen. But on interposing the violet gla.s.s, the design instantly flashes forth in strong contrast with the deep surrounding violet.

President Morton has prepared for me a most beautiful example of such a design which, when placed in the violet light, exhibits a peculiarly brilliant fluorescence. From the experiments of Drs. Bence Jones and Dupre, it would seem that there is some substance in the human body resembling the sulphate of quinine, which causes all the tissues of the body to be more or less fluorescent. All animal infusions show this fluorescence. The crystalline lens of the eye exhibits the effect in a very striking manner. When, for example, I plunge my eye into this violet beam, I am conscious of a whitish-blue shimmer filling the s.p.a.ce before me. This is caused by fluorescent light generated in the eye itself. Looked at from without, the crystalline lens at the same time is seen to gleam vividly.

Long before its physical origin was understood this fluorescent light attracted attention. Boyle describes it with great fulness and exactness. 'We have sometimes,' he says, 'found in the shops of our druggists certain wood which is there called _Lignum Nephritic.u.m,_ because the inhabitants of the country where it grows are wont to use the infusion of it, made in fair water, against the stone in the kidneys. This wood may afford us an experiment which, besides the singularity of it, may give no small a.s.sistance to an attentive considerer towards the detection of the nature of colours. Take _Lignum, Nephritic.u.m_, and with a knife cut it into thin slices: put about a handful of these slices into two or three or four pounds of the purest spring water. Decant this impregnated water into a gla.s.s phial; and if you hold it directly between the light and your eye, you shall see it wholly tinted with an almost golden colour. But if you hold this phial from the light, so that your eye be placed betwixt the window and the phial, the liquid will appear of a deep and lovely ceruleous colour.'

'These,' he continues, 'and other phenomena which I have observed in this delightful experiment, divers of my friends have looked upon, not without some wonder; and I remember an excellent oculist, finding by accident in a friend's chamber a phial full of this liquor, which I had given that friend, and having never heard anything of the experiment, nor having anybody near him who could tell him what this strange liquor might be, was a great while apprehensive, as he presently afterwards told me, that some strange new distemper was invading his eyes. And I confess that the unusualness of the phenomenon made me very solicitous to find out the cause of this experiment; and though I am far from pretending to have found it, yet my enquiries have, I suppose, enabled me to give such hints as may lead your greater sagacity to the discovery of the cause of this wonder.'[21]

Goethe in his 'Farbenlehre' thus describes the fluorescence of horse-chestnut bark:--'Let a strip of fresh horse-chestnut bark be taken and clipped into a gla.s.s of water; the most perfect sky-blue will be immediately produced.'[22] Sir John Herschel first noticed and described the fluorescence of the sulphate of quinine, and showed that the light proceeded from a thin stratum of the solution adjacent to the surface where the light enters it. He showed, moreover, that the incident beam, although not sensibly weakened in luminous intensity, lost, in its transmission through the solution of sulphate of quinine, the power of producing the blue fluorescent light. Sir David Brewster also worked at the subject; but to Professor Stokes we are indebted not only for its expansion, but for its full and final explanation.

-- 3. _The Heat of the Electric Beam. Ignition through a Lens of Ice.

Possible Cometary Temperature_.

But the waves from our incandescent carbon-points appeal to another sense than that of vision. They not only produce light, but heat, as a sensation. The magnified image of the carbon-points is now upon the screen; and with a suitable instrument the heating power of the rays which form that image might be readily demonstrated. In this case, however, the heat is spread over too large an area to be very intense.

Drawing out the camera lens, and causing a movable screen to approach the lamp, the image is seen to become smaller and smaller; the rays at the same time becoming more and more concentrated, until finally they are able to pierce black paper with a burning ring. Pushing back the lens so as to render the rays parallel, and receiving them upon a concave mirror, they are brought to a focus; paper placed at that focus is caused to smoke and burn. Heat of this intensity may be obtained with our ordinary camera and lens, and a concave mirror of very moderate power.