To the eye of a person who understands these things, nothing can be more interesting than the rippling of water under certain circ.u.mstances. By the action of interference its surface is sometimes shivered into the most beautiful mosaic, shifting and trembling as if with a kind of visible music. When the tide advances over a sea-beach on a calm and sunny day, and its tiny ripples enter, at various points, the clear shallow pools which the preceding tide had left behind, the little wavelets run and climb and cross each other, and thus form a lovely _chasing_, which has its counterpart in the lines of light converged by the ripples upon the sand underneath. When waves are skilfully generated in a vessel of mercury, and a strong light reflected from the surface of the metal is received upon a screen, the most beautiful effects may be observed. The shape of the vessel determines, in part, the character of the figures produced; in a circular dish of mercury, for example, a disturbance at the centre propagates itself in circular waves, which after reflection again encircle the centre. If the point of disturbance be a little removed from the centre, the intersections of the direct and reflected waves produce the magnificent chasing shown in the annexed figure (16), which I have borrowed from the excellent work on Waves by the Messrs. Weber. The luminous figure reflected from such a surface is exceedingly beautiful. When the mercury is lightly struck by a gla.s.s point, in a direction concentric with the circ.u.mference of the vessel, the lines of light run round the vessel in mazy coils, interlacing and unravelling themselves in the most wonderful manner. If the vessel be square, a splendid mosaic is produced by the crossing of the direct and reflected waves. Description, however, can give but a feeble idea of these exquisite effects;--

"Thou canst not wave thy staff in the air, Or dip thy paddle in the lake, But it carves the brow of beauty there, And the ripples in rhymes the oar forsake."

[Sidenote: CHASING PRODUCED BY WAVES.]

[Ill.u.s.tration: Fig. 16. Chasing produced by waves.]

[Sidenote: EFFECT OF r.e.t.a.r.dATION.]

Now, all that we have said regarding the r.e.t.a.r.dation of the waves of water, by a whole undulation and a semi-undulation, is perfectly applicable to the case of light. Two luminous points may be placed near to each other so as to resemble the two stones dropped into the water; and when the light of these is properly received upon a screen, or directly upon the retina, we find that at some places the action of the rays upon each other produces darkness, and at others augmented light.

The former places are those where the rays emitted from one point are an _odd_ number of semi-undulations in advance of the rays sent from the other; the latter places are those where the difference of path described by the rays is either nothing, or an _even_ number of semi-undulations. Supposing _a_ and _b_ (Fig. 17) to be two such sources of light, and S R a screen on which the light falls; at a point _l_, equally distant from _a_ and _b_, we have _light_; at a point _d_, where _a d_ is half an undulation longer than _b d_, we have darkness; at _l'_, where _a l'_ is a whole wave-length, or two semi-undulations, longer than _b l'_, we again have light; and at a point _d'_, where the difference is three semi-undulations, we have darkness; and thus we obtain a series of bright and dark s.p.a.ces as we recede laterally from the central point _l_.

[Ill.u.s.tration: Fig. 17. Diagram explanatory of Interference.]

Let a bit of tin foil be closely pasted upon a piece of gla.s.s, and the edge of a penknife drawn across the foil so as to produce a slit.

Looking through this slit at a small and distant light, we find the light spread out in a direction at right angles to the slit, and if the light looked at be _monochromatic_, that is, composed of a single colour, we shall have a series of bright and dark bars corresponding to the points at which the rays from the different points of the slit alternately coincide and interfere upon the retina. By properly drawing a knife across a sheet of letter-paper a suitable slit may also be obtained; and those practised in such things can obtain the effect by looking through their fingers or their eyelashes.

[Ill.u.s.tration: INTERFERENCE SPECTRA, PRODUCED BY DIFFRACTION.

Fig. 18. _To face_ p. 235.]

[Sidenote: CHROMATIC EFFECTS.]

But if the light looked at be white, the light of a candle for example, or of a jet of gas, instead of having a series of bright and dark bars, we have the bars _coloured_. And see how beautifully this harmonizes with what has been already said regarding the different lengths of the waves which produce different colours. Looking again at Fig. 17 we see that a certain obliquity is necessary to cause one ray to be a whole undulation in advance of the other at the point _l'_; but it is perfectly manifest that the obliquity must depend upon the length of the undulation; a long undulation would require a greater obliquity than a short one; red light, for example, requires a greater obliquity than blue light; so that if the point _l'_ represents the place where the first bar of red light would be at its maximum strength, the maximum for blue would lie a little to the left of _l'_; the different colours are in this way separated from each other, and exhibit themselves as distinct fringes when a distant source of white light is regarded through a narrow slit.

By varying the shape of the aperture we alter the form of the chromatic image. A circular aperture, for example, placed in front of a telescope through which a point of white light is regarded, is seen surrounded by a concentric system of coloured rings. If we multiply our slits or apertures the phenomena augment in complexity and splendour. To give some notion of this I have copied from the excellent work of M. Schwerd the annexed figure (Fig. 18) which represents the gorgeous effect observed when a distant point of light is looked at through two gratings with slits of different widths.[B] A bird's feather represents a peculiar system of slits, and the effect observed on properly looking through it is extremely interesting.

[Sidenote: COLOURS OF THIN FILMS.]

There are many ways by which the r.e.t.a.r.dation necessary to the production of interference is effected. The splendid colours of a soap-bubble are entirely due to interference; the beam falling upon the transparent film is partially reflected at its outer surface, but a portion of it enters the film and is reflected at its _inner_ surface. The latter portion having crossed the film and returned, is r.e.t.a.r.ded, in comparison with the former, and, if the film be of suitable thickness, these two beams will clash and extinguish each other, while another thickness will cause the beams to coincide and illuminate the film with a light of greater intensity. From what has been said it must be manifest that to make two red beams thus coincide a thicker film would be required than would be necessary for two blue or green beams; thus, when the thickness of the bubble is suitable for the development of red, it is not suitable for the development of green, blue, &c.; the consequence is that we have different colours at different parts of the bubble. Owing to its compactness and to its being shaded by a covering of debris from the direct heat of the sun, the ice underneath the moraines of glaciers appears sometimes of a pitchy blackness. While cutting such ice with my axe I have often been surprised and delighted by sudden flashes of coloured light which broke like fire from the ma.s.s. These flashes were due to internal rupture, by which fissures were produced as thin as the film of a soap-bubble; the colours being due to the interference of the light reflected from the opposite sides of the fissures.

If spirit of turpentine, or olive oil, be thrown upon water, it speedily spreads in a thin film over the surface, and the most gorgeous chromatic phenomena may be thus produced. Oil of lemons is also peculiarly suited to this experiment. If water be placed in a tea-tray, and light of sufficient intensity be suffered to fall upon it, this light will be reflected from the upper and under surfaces of the film of oil, and the colours thus produced may be received upon a screen, and seen at once by many hundred persons. If the oil of cinnamon be used, fine colours are also obtained, and the breaking up of this film exhibits a most interesting case of molecular action. By using a kind of varnish, instead of oil, Mr. Delarue has imparted such tenacity to these films that they may be removed from the water on which they rest and preserved for any length of time. By such films the colours of certain beetles, and of the wings of certain insects, may be accurately imitated; and a rook's feather may be made to shine with magnificent iridescences. The colours of tempered metals, and the beautiful metallochrome of n.o.bili are also due to a similar cause.

[Sidenote: DIFFRACTION.]

These colours are called the colours of _thin plates_, and are distinguished in treatises on optics from the coloured bars and fringes above referred to, which are produced by _diffraction_, or the bending of the waves round the edge of an object. One result of this bending, which is of interest to us, was obtained by the celebrated Thomas Young.

Permitting a beam of sunlight to enter a dark room through an aperture made with a fine needle, and placing in the path of the beam a bit of card one-thirtieth of an inch wide, he found the shadow of this card, or rather the line on which its shadow might be supposed to fall, always _bright_; and he proved the effect to be due to the bending of the waves of ether round the two edges of the card, and their coincidence at the other side. It has, indeed, been shown by M. Poisson, that the centre of the shadow of a small circular opaque disk which stands in the way of a beam diverging from a point is exactly as much illuminated as if the disk were absent. The singular effects described by M. Necker in the letter quoted at page 178 at once suggest themselves here; and we see how possible it is for the solar rays, in grazing a distant tree, so to bend round it as to produce upon the retina, where shadow might be expected, the impression of a tree of light.[C] Another effect of diffraction is especially interesting to us at present. Let the seed of lycopodium be scattered over a gla.s.s plate, or even like a cloud in the air, and let a distant point of light be regarded through it; the luminous point will appear surrounded by a series of coloured rings, and when the light is intense, like the electric or the Drummond light, the effect is exceedingly fine.

[Sidenote: CLOUD IRIDESCENCE, ETC., EXPLAINED.]

And now for the application of these experiments. I have already mentioned a series of coloured rings observed around the sun by Mr.

Huxley and myself from the Rhone glacier; I have also referred to the cloud iridescences on the Aletschhorn; and to the colours observed during my second ascent of Monte Rosa, the magnificence of which is neither to be rendered by pigments nor described in words. All these splendid phenomena are, I believe, produced by diffraction, the vesicles or spherules of water in the case of the cloud acting the part of the sporules in the case of the lycopodium. The coloured fringe which surrounds the _Spirit of the Brocken_, and the spectra which I have spoken of as surrounding the sun, are also produced by diffraction. By the interference of their rays in the earth's atmosphere the stars can momentarily quench themselves; and probably to an intermittent action of this kind their twinkling, and the swift chromatic changes already mentioned, are due. Does not all this sound more like a fairy tale than the sober conclusions of science? What effort of the imagination could transcend the realities here presented to us? The ancients had their spheral melodies, but have not we ours, which only want a sense sufficiently refined to hear them? Immensity is filled with this music; wherever a star sheds its light its notes are heard. Our sun, for example, thrills concentric waves through s.p.a.ce, and every luminous point that gems our skies is surrounded by a similar system. I have spoken of the rising, climbing and crossing of the tiny ripples of a calm tide upon a smooth strand; but what are they to those intersecting ripples of the "uncontinented deep" by which Infinity is engine-turned!

Crossing solar and stellar distances, they bring us the light of sun and stars; thrilled back from our atmosphere, they give us the blue radiance of the sky; rounding liquid spherules, they clash at the other side, and the survivors of the tumult bear to our vision the wondrous cloud-dyes of Monte Rosa.

FOOTNOTES:

[A] The vibrations of the air of a room in which a musical instrument is sounded may be made manifest by the way in which fine sand arranges itself upon a thin stretched membrane over which it is strewn; and indeed Savart has thus rendered visible the vibrations of the tympanum itself. Every trace of sand was swept from a paper drum held in the clock-tower of Westminster when the Great Bell was sounded. Another way of showing the propagation of aerial pulses is to insert a small gas jet into a vertical gla.s.s tube about a foot in length, in which the flame may be caused to burn tranquilly. On pitching the voice to the note of an open tube a foot long, the little flame quivers, stretches itself, and responds by producing a clear melodious note of the same pitch as that which excited it. The flame will continue its song for hours without intermission.

[B] I am not aware whether in his own country, or in any other, a recognition at all commensurate with the value of the performance has followed Schwerd's admirable essay ent.i.tled 'The Phenomena of Diffraction deduced from the Theory of Undulation.'

[C] I think, however, that the strong irradiation from the glistening sides of the twigs and branches must also contribute to the result.

[Sidenote: RADIANT HEAT.]

(2.)

Thus, then, we have been led from Sound to Light, and light now in its turn will lead us to _Radiant Heat_; for in the order in which they are here mentioned the conviction arose that they are all three different kinds of motion. It has been said that the beams of the sun consist of rays of different colours, but this is not a complete statement of the case. The sun emits a mult.i.tude of rays which are perfectly non-luminous; and the same is true, in a still greater degree, of our artificial sources of illumination. Measured by the quant.i.ty of heat which they produce, 90 per cent. of the rays emanating from a flame of oil are obscure; while 99 out of every 100 of those which emanate from an alcohol flame are of the same description.[A]

[Sidenote: OBSCURE RAYS.]

In fact, the visible solar spectrum simply embraces an interval of rays of which the eye is formed to take cognizance, but it by no means marks the limits of solar action. Beyond the violet end of the spectrum we have obscure rays capable of producing chemical changes, and beyond the red we have rays possessing a high heating power, but incapable of exciting the impression of light. This latter fact was first established by Sir William Herschel, and it has been amply corroborated since.

The belief now universally prevalent is, that the rays of heat differ from the rays of light simply as one colour differs from another. As the waves which produce red are longer than those which produce yellow, so the waves which produce this obscure heat are longer than those which produce red. In fact, it may be shown that the longest waves never reach the retina at all; they are completely absorbed by the humours of the eye.

What is true of the sun's obscure rays is also true of calorific rays emanating from any obscure source,--from our own bodies, for example, or from the surface of a vessel containing boiling water. We must, in fact, figure a warm body also as having its particles in a state of vibration.

When these motions are communicated from particle to particle of the body the heat is said to be _conducted_; when, on the contrary, the particles transmit their vibrations through the surrounding ether, the heat is said to be _radiant_. This radiant heat, though obscure, exhibits a deportment exactly similar to light. It may be refracted and reflected, and collected in the focus of a mirror or of a suitable lens.

The principle of interference also applies to it, so that by adding heat to heat we can produce _cold_. The ident.i.ty indeed is complete throughout, and, recurring to the a.n.a.logy of sound, we might define this radiant heat to be light of too low a pitch to be visible.

I have thus far spoken of _obscure_ heat only; but the selfsame ray may excite both light and heat. The red rays of the spectrum possess a very high heating power. It was once supposed that the heat of the spectrum was an essence totally distinct from its light; but a profounder knowledge dispels this supposition, and leads us to infer that the selfsame ray, falling upon the nerves of feeling, excites heat, and falling upon the nerves of seeing, excites light. As the same electric current, if sent round a magnetic needle, along a wire, and across a conducting liquid, produces different physical effects, so also the same agent acting upon different organs of the body affects our consciousness differently.

FOOTNOTES:

[A] Melloni.

(3.)

[Sidenote: HEAT A KIND OF MOTION.]

Heat has been defined in the foregoing section as a motion of the molecules or atoms of a body; but though the evidence in favour of this view is at present overwhelming, I do not ask the reader to accept it as a certainty, if he feels sceptically disposed. In this case, I would only ask him to accept it as a symbol. Regarded as a mere physical image, a kind of paper-currency of the mind, convertible, in due time, into the gold of truth, the hypothesis will be found exceedingly useful.

All known bodies possess more or less of this molecular motion, and all bodies are communicating it to the ether in which they are immersed. Ice possesses it. Ice before it melts attains a temperature of 32 Fahr., but the substance in winter often possesses a temperature far below 32, so that in rising to 32 it is _warmed_. In experimenting with ice I have often had occasion to cool it to 100 and more below the freezing point, and to warm it afterwards up to 32.

If then we stand before a wall of ice, the wall radiates heat to us, and we also radiate heat to it; but the quant.i.ty which we radiate being greater than that which the ice radiates, we lose more than we gain, and are consequently chilled. If, on the contrary, we stand before a warm stove, a system of exchanges also takes place; but here the quant.i.ty we receive is in excess of the quant.i.ty lost, and we are warmed by the difference.

In like manner the earth radiates heat by day and by night into s.p.a.ce, and against the sun, moon, and stars. By day, however, the quant.i.ty received is greater than the quant.i.ty lost, and the earth is warmed; by night the conditions are reversed; the earth radiates more heat than is sent to her by the moon and stars, and she is consequently cooled.

But here an important point is to be noted:--the earth receives the heat of the sun, moon, and stars, in great part as _luminous_ heat, but she gives it out as _obscure_ heat. I do not now speak of the heat reflected by the earth into s.p.a.ce, as the light of the moon is to us; but of the heat which, after it has been absorbed by the earth, and has contributed to warm it, is radiated into s.p.a.ce, as if the earth itself were its independent source. Thus we may properly say that the heat radiated from the earth is _different in quality_ from that which the earth has received from the sun.

[Sidenote: QUALITIES OF HEAT.]