Treatise on Light

movement is caused by the violent shaking of the particles of metal or wood, of which those on the surface strike the ethereal matter in the same way. Furthermore, the agitation of the particles that generate light must be much swifter and more rapid than that of the bodies which cause sound, since we do not see that the trembling of a sounding body is capable of giving rise to light, just as the movement of the hand in the air is not capable of producing Sound.

Now, if one examines what this matter might be in which the movement coming from luminous bodies spreads—which I call EtherealHuygens refers to the "Aether," a hypothetical substance once thought to fill all space and allow light waves to travel through a vacuum.—one will see that it is not the same matter that serves for the propagation of Sound. For we find that the latter is properly this air that we feel and that we breathe, which, being removed from a place, does not prevent the other matter that serves light from being found there. This is proven by enclosing a sounding body in a glass vessel, from which the air is then drawn by the machine that Mr. BoyleRobert Boyle (1627–1691), the Anglo-Irish natural philosopher who improved the air pump to create vacuums for his experiments. has given us, and with which he has performed so many beautiful experiments. But in performing the one of which I speak, one must take care to place the sounding body on cotton or on feathers, so that it cannot communicate its vibrations to the glass vessel that encloses it, nor to the machine—a detail which had until now been neglected. For then, after having emptied all the air, one hears no Sound from the metal even though it is struck.

One sees from this not only that our air, which does not penetrate the glass, is the matter through which Sound spreads; but also that it is not this same air, but another matter in which light spreads; since, the air being removed from this vessel, the light does not cease to pass through it as before.

And this last point is demonstrated even more clearly by