On the Nature of Sounds.

Fourth. They have certain differences, as is evident from the light of the sun compared to a candle, and from the diversity of musical tones.

Fifth. That which they produce in their sphere of activity original: "sphæra actiuitatis," referring to the physical range where an effect can be felt is not at all visible to the eyes when it consists of air, for air does not stop the rays of light, nor does the motion of that same air strike the eyes.

Sixth. When either is greater, it overshadows the smaller, which seems to be extinguished. This is evident from a candle flame exposed to midday light, and from a low voice joined to the clangor of a trumpet or the crash of thunder.

Seventh. When either rushes into the sense more violently than is fitting, such as the sun into the eye or the crash of thunder into the ear, it destroys or damages the sense.

Eighth. Both can be reflected, as is evident from burning mirrors concave mirrors used to focus sunlight to start fires and from those things which create echoes.

Ninth. Both act upon living things as well as those lacking life. Light moves the sight and burns; sound, however, moves the hearing and strings original: "fides," referring to the strings of a musical instrument vibrating in sympathy.

Tenth. The effects of both vary according to the differences of the medium. Just as light seen through smoke appears to tremble, so sound passing through winds is seen to be depressed one moment and raised the next.

Eleventh. Air is considered to be the most suitable medium for both.

Twelfth. Both seem to produce heat by the thinning of the air original: "rarefactione," a term for air becoming less dense. This happens because of the gathering of rays or the multiplication of sounds and the motion of the air. But truly, any analogy or similarity of light and sound deserves its own special proposition, which others will be able to establish. For now, having omitted other analogies, I come to the differences between the two.

PROPOSITION VI.

To explain the differences between light and sound.

It is certain, first, that light is spread into its sphere of activity in a shorter time than sound. This is evident from the flash of cannons original: "tormentorum bellicorum" and sulfur, which are seen from afar long before the crash of the cannon or the thunder is heard. The reason is that sound is motion, or happens with motion, which cannot occur in an instant. Yet an instant seems to be the measure for the production of light expanded into its whole sphere. I said "seems," because it is not so certain that light is spread through the whole free medium without some doubting whether there is a motion of light that a new sense might detect, surpassing the speed of the eye and sight in the same way that sight surpasses hearing.

Second, the communication or diffusion of sound is more hindered than that of light by the motions of the air and by winds. When the winds are opposing, that diameter of the sphere of sound which has contrary winds is much shorter than the other semi-diameter which has favoring winds. However, each semi-diameter of the sphere of light is always equal, no matter which winds are blowing.

Third, sounds are carried to the ear not only in a straight line or direct rays, but also in circular, elliptical, parabolic, and any other lines. Light, however, is carried to the eye only in straight lines, whether they are direct, reflected, or refracted original: "refractis," the bending of light as it passes through different materials.

Fourth, sounds are hindered by hard bodies no matter how transparent original: "diaphanis" they are, such as crystal, glass, and the like, just as much as by opaque bodies of the same density. Yet light is hindered little or not at all by them. Therefore, light is perhaps spread more easily where transparent bodies are denser, provided they are perfectly clear. If, however, the bodies are thin but opaque, sound seems to be less hindered than a ray of light. This is evident from cloth, carpets, paper, and the like, which bring a far greater hindrance to light than to sound to prevent it from going further.

Fifth, very small bodies reflect light perceptibly, as is evident from a mirror whose diameter is one royal inch, which burns. But a larger body seems to be required to reflect a perceptible sound, that is, to produce an echo, although there is no doubt that sound is reflected by any hard body.

Sixth, light and other visible things are seen from a greater distance than sounds are heard. By what ratio a sound is given for a specific space, and vice versa, will be investigated elsewhere.

Seventh, a visible thing requires a greater distance from the eye than a sound does from the ear so that it can be seen correctly. Whether any visible thing can be as unpleasant to the eye as a sound can be displeasing to the hearing is for those to consider who have imagined or sometimes perceived images of the most unpleasant sounds and horrific sights. I think death can be brought to living creatures by both, as can be confirmed from seen ghosts and from the crash of cannons and thunder, about which we shall say more later. The remaining differences, like the analogies, will be easily concluded from what is to be said.

Furthermore, many other properties and analogies of light and sound may be read, and if it pleases, transferred here from the second book of our French Music Referring to Mersenne's earlier work, Harmonie Universelle, in which sounds are treated most extensively.

PROPOSITION VII.

Sounds do not depend on the body from which they were first produced.

Indeed, they have this in common with many other effects. The motion of a thrown stone does not depend on the thrower in such a way that it stops when the thrower stops. Nor, if a cannon should vanish into nothing, would the crash that began before the perfect period of its duration end; rather, it will be heard through the whole sphere of, for example, ten leagues. But concerning the size of the sphere of a given sound, more will be said later. Light, however, depends on the shining body not only in its becoming, as they say, but also so that it may be preserved. For at the same moment the candle is withdrawn, all the light spread through the room or sphere perishes. However, some assert they have seen a stone which, having conceived light, keeps it for some time in the dark; concerning this, see La Galla's treatise On Light Giulio Cesare della Galla (1571–1624), an Italian philosopher.

And from this, it is clear that sounds cannot be enclosed in a box so that they might be heard later by its opening. But why...