SCALE OF ACTUAL WAVE LENCTHS OF SPECTRUM IN 100000THS OF A MILLIMETRE

THE QUARTERLY

JOURNAL OF SCIENCE.

JANUARY, 1870.

I. LIGHT AND SOUND:

AN EXAMINATION OF THEIR REPUTED ANALOGY.

By W. F. BARRETT, F.C.S., Natural Science Master at the London International College, &c.

LONG before we knew anything of the origin either of sound or light, the existence of an analogy between these forces had been the subject of speculation by some philosophers. But the idea of such an analogy did not originate in philosophy; it was not confined to a few; it resulted in more than speculation. From the earliest times we find among all nations a crude perception of a similarity between sound and colour. This perception became rooted in their languages. The same words, in many cases, were employed to denote either light, or sound. A vivid impression received by the eye was equivalent to a forcible shock received by the ear: thus, the English "loud," the French "criard," the German "schreiend,' are identical expressions, relating to sound, also applied to glaring colours. Faintness of vision and feebleness of voice were spoken of as one. Our own words dim and dumb were probably cognate terms in Anglo-Saxon.

It is easy to trace this correspondence in language much farther, but that is not our present business. Let us inquire if this widespread mental analogy between sound and colour rests upon a physical basis. Is it true that light and sound are alike, and if so in what way are they alike? How can the swift flash from a gun be said to resemble the sluggish report that follows? Except æsthetically, where is the likeness between a painting by Raphael and a theme by Beethoven?

At the outset let us remark that no attempt will be made to show the identity of light and sound: it is their resemblance, their parallelism, and not, of course, their oneness we wish to establish. A parallelism that probably is metaphysical as well as physical; so that the estimation of beauty of colouring and harmony of sound may, hereafter, be found to resolve themselves into mental actions

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essentially the same. Here, however, we have solely to deal with the physical aspect of the question. In pursuit of our object it will be necessary to compare the principal phenomena of light and sound, and for this purpose it will be convenient to break up the subject into sections. If the analogy be just, it will assuredly gather strength as the comparison proceeds; if it be false, then each section cannot fail to force this fact upon the mind. In either case the result ought to be profitable, if we simply seek the truth.

§ 1. ORIGIN OF LIGHT AND Sound.

Light and sound are both the products of vibratory motion. But to evolve light the motion must be enormously swift, whilst to produce sound the motion must be comparatively slow. In the former case only impalpable molecules can be made to attain the requisite swiftness-light is therefore a molecular motion of vibration. In the latter case visible masses of matter can be moved to and fro with the necessary speed: sound is therefore usually the product of a molar motion of vibration. Further, to continue the light, or to sustain the sound, the to-and-fro motion must be performed in equal times; it must be isochronous. If not isochronous the light will be either intermittent or varying in colour, and the sound will be either a noise or musical notes of varying pitch.

Now comes a remarkable point. Sound and even music are usually produced by a disturbance very different from a vibratory motion. If we hit a tuning-fork on our knee, strike the strings of a piano, or pluck the strings of a harp, we produce music by rough mechanical means; so the noise of hammering, the roar of cataracts, the whistling of the wind, or "the scream of a maddened beach,' are all sounds, that is motions of vibration, produced by a rude motion of translation. Light, also, can be evolved by similar agency. The rubbing of two pieces of quartz or sugar, the sparks from a flint or steel, and the incandescence produced by the friction of meteors against the air are familiar examples of light generated by mechanical means.

How can we account for the transition from molar to molecular motion, from an impulse, such as a blow, to a regular pendulum-like swing? A well-established law of mechanics is no doubt the true explanation. This law may be stated as follows:-"That if a body receive a shock and sufficient time be allowed to elapse so that the initial disturbance is destroyed by friction, imperfect elasticity and other causes, the final resultant motion will be vibratory and isochro

*This appears to be a fundamental law of the universe; namely, that an original impulse of any kind finally resolves itself into periodic or rythmic motion. Does not this throw light upon the periodic motion of planets as well as the vibratory motion of atoms? Possibly, in some such way, we may hereafter learn to understand the musical rôle of nature.

nous." Here, then, we have at once the explanation, and the analogy of the mechanical origin of luminous and sonorous vibrations.

It is possible, however, to produce sound as well as light by a molecular motion of vibration. This is the case when a wire or rod is rubbed longitudinally; and, as might be predicted, the sound thus obtained is far more shrill than when the same body vibrates transversely. Here, indeed, in longitudinal vibration, there are some striking points of contact between light and sound. For if a beam of polarized light be transmitted through a strip of glass, as soon as the glass, by rubbing it longitudinally, is caused to emit a sound, the light is powerfully affected.

Light and sound have, further, a common origin in molecular change when they are generated by chemical action; thus light and more or less of sound attends combustion; and, directly or indirectly, sound and more or less of light attend the explosion of fulminating powders. Again, light is produced by electricity, as in lightning or the electric spark and electric light, and sound simultaneously accompanies each of these phenomena. Light attends the quick evolution of heat, and sudden heat is productive of the loudest sounds, as in the explosion of mixed oxygen and hydrogen gases.

Now, as light and sound are both the products of motion, the law of the conservation of force teaches us that neither one nor the other can have been produced without the loss of an equivalent amount of motion, that is force, elsewhere; and, moreover, that neither can have disappeared without the production of an equivalent motion or force of another kind. This being so, the doctrine of the correlation, or mutual convertibility, of the physical forces comes in and shows us the possibility, not at once perhaps but through intermediate steps, of exchanging light for sound and sound for light. As it is, already we know that the quenching of both light and sound, by absorbing media, results in the production of the same mode of motion, namely, that which we designate heat.

§ 2. THE PROPAGATION OF LIGHT AND SOUND.

Originating in vibratory motion, let us now inquire in what manner the forces of light and sound reach the eye and ear respectively. When a stick is allowed to swing to and fro in still water, the motion is communicated to the medium around, and a series of waves travel outward from the centre of disturbance. The motion, or vis viva, of the stick, though retarded and finally brought to rest by the friction of the water, is not lost. The movement has been delivered to the water, and part of it has reappeared in the form of the waves we noticed. The same considerations apply to light and sound. When a bell is struck its vibrations are delivered to the air around; a system of aerial pulses or waves is thus generated,

which reaching the ear give rise to the sensation of sound. When a substance is made incandescent, the luminous vibrations do not throw the air into undulation; a finer and more elastic medium is requisite to accept and carry on molecular motion. We have abundant reason to believe that such a medium exists; we term it the luminiferous ether. To this ether, then, the luminous body communicates its motion. Here, also, a system of waves is produced which, striking the eye, finally give birth to the sensation of light. There is, however, this difference between the sound-waves of the air and the light-waves of the ether, that whereas the former move to and fro longitudinally, the latter vibrate transversely. A cornfield ruffled with gusts of wind exhibits waves like those of sound; the water of a lake thrown into ripples by a disturbance exhibits waves like those of light. The upper figure, L, in the accompany drawing (Fig. 1) shows a single plane-wave of light: the

lower, s, a single wave of sound. Each has at N its node, or place of rest, and at v its vibrating segment, or place of greatest motion. The sizes of the two waves are vastly disproportionate. The average length of a sonorous wave, say that of the middle c in the piano, is about 50 inches from N to N'. The average length of a luminous wave, say that of green light, is only the 100th part of an inch from N to N'. This great difference must not stagger us in tracing out our analogy. The element of size does not enter into the region of law. The truth of this statement will become evident as we see laws obeyed in like manner by light and sound; by undulations, one set of which are millions of times the size of the other.

Another strong point in this analogy is presented by the phenomena of Interference. From an inspection of Fig. 1, it will readily be seen how this is produced. Let L be a series of waves; suppose a second series, of exactly the same length as L, to start just half an undulation later; it will be clear that the crest of one set will coincide with the hollow of the other. They will thus mutually destroy each other, and darkness will be the product of