Transactions and Proceedings of the New Zealand Institute/Volume 25/Article 74

by Annette Wilson

Art. LXXIV.—Analogy between Light and Sound: Are they Convertible?

By Miss Annette Wilson.

[Read before the Otago Institute, 11th October, 1892.]

That a certain analogy exists between light and sound has long been a recognised fact, and more or less commented on. But that that analogy should be so complete as to argue an affinity between them—nay, more, that it might even be possible to convert the one into the other—this is by no means so generally admitted.

In this idea, which I have long entertained, it would appear that I am not alone, as only a short time ago mention was made in one of the daily papers that Edison purposed making us hear noises in the sun, and this was to be effected by converting the rays forming the spectrum into sound.

It is my purpose to-night to illustrate this idea, first directing your attention to some of the remarkable analogies existing between light and sound, and then translating sound into colour, by what I believe to be a novel process or experiment, using for that purpose coloured glasses.

We know that both light and sound can be converted into heat. We also know that light, sound, and radiant heat are analogous in their laws and conditions. Professor Tyndall remarks upon this in his work upon "Sound." He says, "The action of sound is exactly the same as that of light and radiant heat. They, like sound, are wave-motions; like sound they diffuse themselves in space, diminishing in intensity according to the same law; in fact, every experiment on the refraction of light has its analogue in the refraction of sound."

Does not this alone portray the affinity of light, sound, and radiant heat, and suggest the idea that ultimately they may be found to be but different manifestations of one and the same thing, as we see illustrated by ice, water, and snow?

Let us now briefly consider the way in which light and sound are conveyed to our senses, that we may the better trace their analogy.

Light and sound, as we know, are not substances, but only the vibrations of substances, which vibrations are conveyed to our senses by pulsations through a connecting medium. Light reaches the eye by pulsations through ether, which pervades space; sound reaches the ear by pulsations through atmospheric air.

First, as to sound. Sixteen pulsations in a second are necessary to produce the lowest musical sound. Below that number, the pulsations, being distinctly heard, form no musical sound, only noise. The highest musical sound which our ears are capable of receiving requires about forty thousand pulsations in a second. Above that number a shrill noise or whistle is the result. Now, if we take the seven notes of any diatonic scale we shall find that the number of pulsations producing the seventh note, counting upwards, is all but double the number required to produce the lowest or first note. Thus, middle C on the piano requires 258.7 pulsations in a second; and the seventh note of the ascending scale of C—viz., B—takes 488.2 pulsations. The octave of any note takes exactly double the number of the first; therefore the octave of middle C takes 517.4. This is the pitch adopted by the Paris Conservatoire, and quoted by Deschanel in his work on "Sound and Light." There are three prominent notes in the scale, which form what is called the "common chord"; these notes are the first or keynote, the third or mediant, and the fifth or dominant. These sounds can be produced by striking any key on the piano very forcibly, with the loud pedal down, when the third and fifth of that note will be heard sounding faintly after it. In the scale of C, for instance, C, E, G, form the common chord. This satisfies the ear by itself, and is called a concord, whereas combinations of other notes in the scale are not so satisfactory to the ear, and others, again, produce complete discord.

Let us now turn to the consideration of how light is conveyed to the eye through the medium of ether.

The vibrations of light are exceedingly rapid. The lowest number that can make any impression on the retina of the eye as light is computed to be about four hundred and fifty billions in a second. This produces the sensation of red, which is the lowest colour in the spectrum. The highest colour, ultra-violet, requires about eight hundred billions, which, you will observe, is nearly double the number required for the formation of red. Above eight hundred billions in a second only chemical action is the result; below four hundred and fifty billions, heat.

I will here give a passage which Shellan quotes from Dove, in his "Spectrum Analysis," page 65. He says,—

"Dove describes in his own ingenious manner the course of the vibrations as they produce successively sound, heat, and light, as follows: 'In the middle of a large darkened room let us suppose a rod set in vibration, and connected with a contrivance for continually augmenting the speed of the vibrations. I enter the room at the moment when the rod is vibrating four times in a second. Neither eye nor ear tells me of the presence of the rod, only the hand, which feels the strokes when brought within their reach. The vibrations become more rapid, till when they reach the number of thirty-two in a second a deep hum strikes my ear (that is to say, the tympanum is pressed sixteen times, and sixteen times withdrawn, therefore sixteen blows are received upon the ear). The tone rises continually in pitch, and passes through all the intervening grades up to the highest, the shrillest note; then all sinks again into the former grave-like silence. While full of astonishment at what I have heard, I feel suddenly (by the increased velocity of the vibrating-rod) an agreeable warmth, as from a fire, diffusing itself from the spot whence the sound had proceeded. Still all is dark. The vibrations increase in rapidity, and a faint-red light begins to glimmer; it gradually brightens till the rod assumes a vivid red glow, then it turns to yellow, and changes through the whole range of colours up to violet, when all again is swallowed up in night. Thus nature speaks to the different senses in succession—at first a gentle word, audible only in immediate proximity; then a louder call from an ever-increasing distance; till, finally, her voice is borne on the wings of light from regions of immeasurable space.'"

This passage bears out my idea that light and sound are convertible, the one into the other, through the medium of heat.

In another place Shellan says,—

"The gradation of colour from red to ultra-violet is to the eye what the gamut is to the ear, and it is not without reason that we talk of harmony of tone and colour. To the physicist the words 'colour' and 'tone' are only different modes of expression for similar and closely-allied phenomena; they express the perception of regular movements recurring in equal periods of time, in ether producing colours, in air musical sounds, &c."

In tracing the colours in the spectrum from red to ultra-violet we notice first the three primary colours, red, yellow, and blue. Between red and yellow orange is formed; between yellow and blue comes green; and above blue we find violet and ultra-violet, which last seems on the verge of running into the red again, just as the seventh note of the musical scale, or leading note as it is called, seems to want the octave, or repetition of the keynote, to follow it—in other words, it suggests it to the ear.

The colours in the spectrum are arranged as follows: Red, orange, yellow, green, blue, violet, and ultra-violet (some authorities name the last two indigo and violet).

Now I come to a very important analogy. If we take the proportion of increase in the number of vibrations in a second required to produce the seven colours, and if we take the proportion of increase in the number of vibrations in a second required to produce the seven musical sounds of a diatonic scale, we shall find these proportions exactly the same. Some years ago, in England, I attended a lecture entitled "The Correlation of Light and Sound," and the lecturer produced on a screen a table of the ratio of wave-lengths in a second, producing the seven colours in spectrum, and the seven notes of a scale (of course, wave-lengths vary in number in inverse ratio to their velocity). I exhibit the table which I copied at the time.

You will here see the names of the seven colours in the spectrum placed over their relations, the seven notes of a scale, with the ratio of their wave-lengths, thus:—Note C, 100 = red; D, 89 = orange; E, 80 = yellow; F, 75 = green; G, 67—blue; A, 60 = violet; B, 53 = ultra-violet. Any diatonic scale would yield the same proportion of wave-lengths. Of course, the division in both is arbitrary, both sound and light being continuous from their lowest to their highest manifestations.

If we take the primary colours—red, yellow, and blue—in the above table we shall find that they occupy the same position in the spectrum as do the three notes forming the common chord in the scale—viz., the first, third, and fifth. Moreover, any two notes that sound discordant stand under colours that will not harmonize to the eye, and ditto vice versâ.

By the following illustrations on the screen I hope to make you realise the capability of colour to impress the brain in the same way as sound does the ear. I am aware that it is a very crude and inadequate attempt, but the means at my command are limited. In my mind's eye I can see a symphony of Beethoven rendered by electric flashes of coloured light, with all its grand concords and discords, producing the various emotions of peace or agitation, joy or sorrow, triumph or despair. We must remember that music began by simple melodies only, just as to-night I can give you such only in colour. What might not be done in the future?

Slide 1a: Colours in spectrum—red, orange, yellow, green, blue, violet, ultra-violet.

Small frames: Discord and concord.

Discord	Concord
(having three secondary and one primary colour).	(all primary colours).
Orange. d Green. f Blue. g Ultra-violet. b	Red. c Yellow. e Blue. g Red. c

Slide 1b: Four opening bars of Beethoven's Symphony in C minor, opening in E flat major. First four notes all primary, ending on the keynote, complete and satisfactory. Second four all secondary, incomplete and unsatisfactory.

Slide 2: "Scots wha hae."

Slide 3: "Last Rose of Summer."

Slide 4: "Weel may the Keel row."

Slides 5 and 6: "God save the Queen."

N.B.—Only having been able to arrange seven colours with glasses to form one diatonic scale, it has been necessary to transpose each air into that one scale. With the intermediate shades, forming twelve gradations, a chromatic scale would be the result, corresponding to the twelve semitones in the octave, and these airs could be given in their own keys.

The twelve would be as follows:—

Colour .. Note..	1. Red. c	2. Red-orange csharp or dflat	3. Orange. d	4. Yellow-orange. dsharp or eflat
Colour .. Note..	5. Yellow. e	6. Green. f	7. Blue-green. fsharp or gflat	8. Blue. g
Colour .. Note..	9. Blue-violet. gsharp or aflat	10. Violet. g	11. Red-violet. asharp or bflat	12. Ultra-violet b	Red. c