Treatise on Light

OF LIGHT. CHAPTER I.

...passed in 22 minutes, this makes one thousand diameters in a minute, and 16 2/3 diameters in a second or heartbeat The "battement d’artère" (pulse-beat) was a common informal unit for a second in the 17th century., which amounts to more than eleven hundred times one hundred thousand toises toise: an old French unit of length, roughly 1.949 meters or 6.4 feet; since the diameter of the Earth contains 2,865 leagues (at 25 leagues per degree), and each league is 2,282 Toises, according to the exact measurement that Mr. Picard Jean-Félix Picard (1620–1682), the French astronomer who first accurately measured the size of the Earth using triangulation. took by order of the King in 1669. But Sound, as I have said before, only travels 180 toises in the same time of one second: therefore the speed of light is more than six hundred thousand times greater than that of Sound: which, however, is quite different from being instantaneous, since there is the same difference as between something finite and something infinite. Now, the successive movement of light being confirmed in this manner, it follows, as I have already said, that it spreads in spherical waves, just like the movement of Sound.

But if both resemble each other in that respect, they differ in several other things; namely in the initial production of the movement that causes them; in the matter through which this movement spreads; and in the manner in which it is communicated. For as far as the production of Sound is concerned, we know it is by the sudden shaking of an entire body, or of a considerable part, which agitates all the contiguous air. But the movement of light must originate, as it were, from every point of the luminous object, to be able to make all the different parts of that object perceptible, as will be better seen in what follows. And I do not believe that this movement can be better explained than by supposing those among the luminous bodies which are liquid, such as flame, and apparently the sun and the stars, to be composed of particles swimming in a much more subtle matter, which agitates them with great rapidity and makes them strike against the particles of the ether, which surround them and are much smaller than they are. But that in solid luminous bodies such as coal, or metal reddened in the fire, this same move-

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