Musical sounds differ in pitch: some notes are high and shrill, others low and deep. Boys are chosen as choristers to produce the shrill notes; men are chosen to produce the bass notes. Now, the sole difference here is, that the boy's organ vibrates more rapidly than the man's—it sends a greater number of impulses per second to the ear. In like manner, a short string emits a higher note than a long one, because it vibrates more quickly. The greater the number of vibrations which any instrument performs in a given time, the higher will be the pitch of the note produced. The reason why the hum of a gnat is shriller than that of a beetle is that the wings of the small insect vibrate more quickly than those of the larger one. We can, with suitable arrangements, make those sonorous vibrations visible to the eye;[A] and we also possess instruments which enable us to tell, with the utmost exactitude, the number of vibrations due to any particular note. By such instruments we learn that a gnat can execute many thousand flaps of its little wings in a second of time.
NATURE OF LIGHT.
In the study of nature the coarser phenomena, which come under the cognizance of the senses, often suggest to us the finer phenomena which come under the cognizance of the mind; and thus the vibrations which produce sound, and which, as has been stated, can be rendered visible to the eye by proper means, first suggested that light might be due to a somewhat similar action. This is now the universal belief. A luminous body is supposed to have its atoms, or molecules, in a state of intense vibration. The motions of the atoms are supposed to be communicated to a medium suited to their transmission, as air is to the transmission of sound. This medium is called the luminiferous ether, and the little billows excited in it speed through it with amazing celerity, enter the pupil of the eye, pass through the humours, and break upon the retina or optic nerve, which is spread out at the back of the eye. Hence the tremors they produce are transmitted along the nerve to the brain, where they announce themselves as light. The swiftness with which the waves of light are propagated through the ether, is however enormously greater than that with which the waves of sound pass through the air. An aërial wave of sound travels at about the rate of 1100 feet in a second: a wave of light leaves 192,000 miles behind it in the same time.
CAUSE OF COLOUR.
Thus, then, in the case of sound, we have the sonorous body, the air, and the auditory nerve, concerned in the phenomenon; in the case of light, we have the luminous body, the ether, and the optic nerve. The fundamental analogy of sound and light is thus before us, and it is easily remembered. But we must push the analogy further. We know that the white light which comes to us from the sun is made up of an infinite number of coloured rays. By refraction with a prism we can separate those rays from each other, and arrange them in the series of colours which constitute the solar spectrum. The rainbow is an imperfect or impure spectrum, produced by the drops of falling rain, but by prisms we can unravel the white light into pure red, orange, yellow, green, blue, indigo, and violet. Now, this spectrum is to the eye what the gamut is to the ear; each colour represents a note, and the different colours represent notes of different pitch. The vibrations which produce the impression of red are slower, and the waves which they produce are longer, than those to which we owe the sensation of violet; while the vibrations which excite the other colours are intermediate between these two extremes. This, then, is the second grand analogy between light and sound: Colour answers to Pitch. There is therefore truth in the figure when we say that the gentian of the Alps sings a shriller note than the wild rhododendron, and that the red glow of the mountains at sunset is of a lower pitch than the blue of the firmament at noon.
LENGTH OF ETHEREAL WAVES.
These are not fanciful analogies. To the mind of the philosopher these waves of ether are almost as palpable and certain as the waves of the sea, or the ripples on the surface of a lake. The length of the waves, both of sound and light, and the number of shocks which they respectively impart to the ear and eye, have been the subjects of the strictest measurement. Let us here go through a simple calculation. It has been found that 39,000 waves of red light placed end to end would make up an inch. How many inches are there in 192,000 miles? My youngest reader can make the calculation for himself, and find the answer to be 12,165,120,000 inches. It is evident that, if we multiply this number by 39,000, we shall obtain the number of waves of red light in 192,000 miles; this number is 474,439,680,000,000. All these waves enter the eye in one second; thus the expression "I see red colour," strictly means, "My eye is now in receipt of four hundred and seventy-four millions of millions of impulses per second." To produce the impression of violet light a still greater number of impulses is necessary; the wave-length of violet is the 1/57500th part of an inch, and the number of shocks imparted in a second by waves of this length is, in round numbers, six hundred and ninety-nine millions of millions. The other colours of the spectrum, as already stated, rise gradually in pitch from the red to the violet.
A very curious analogy between the eye and ear may here be noticed. The range of seeing is different in different persons—some see a longer spectrum than others; that is to say, rays which are obscure to some are luminous to others. Dr. Wollaston pointed out a similar fact as regards hearing; the range of which differs in different individuals. Savart has shown that a good ear can hear a musical note produced by 8 shocks in a second; it can also hear a note produced by 24,000 shocks in a second; but there are ears in which the range is much more limited. It is possible indeed to produce a sound which shall be painfully shrill to one person, while it is quite unheard by another. I once crossed a Swiss mountain in company with a friend; a donkey was in advance of us, and the dull tramp of the animal was plainly heard by my companion; but to me this sound was almost masked by the shrill chirruping of innumerable insects which thronged the adjacent grass; my friend heard nothing of this, it lay quite beyond his range of hearing.
A third and most important analogy between sound and light is now to be noted; and it will be best understood by reference to something more tangible than either. When a stone is thrown into calm water a series of rings spread themselves around the centre of disturbance. If a second stone be thrown in at some distance from the first, the rings emanating from both centres will cross each other, and at those points where the ridge of one wave coincides with the ridge of another the water will be lifted to a greater height. At those points, on the contrary, where the ridge of one wave crosses the furrow of another, we have both obliterated, and the water restored to its ordinary level. Where two ridges or two furrows unite, we have a case of coincidence; but where a ridge and a furrow unite we have what is called interference. It is quite possible to send two systems of waves into the same channel, and to hold back one system a little, so that its ridges shall coincide with the furrows of the other system. The "interference" would be here complete, and the waves thus circumstanced would mutually destroy each other, smooth water being the result. In this way, by the addition of motion to motion, rest may be produced.
LIGHT ADDED TO LIGHT MAKES DARKNESS.