Treatise on Light

straight, so that they touch one another; one finds, by striking with a similar ball against the first of these balls, that the movement passes as if in an instant to the last one, which separates from the row without any visible movement in the others. Even the ball that struck remains motionless with them. Here we see a transmission of movement of extreme speed, which is all the greater as the material of the balls is of greater hardness.

But it is also certain that this progression of movement is not instantaneous, but successive, and thus it requires time. For if the movement—or, if one prefers, the inclination toward movement—did not pass successively through all these balls, they would all acquire it at the same time, and consequently, they would all move forward together; this does not happen. Instead, the last one leaves the entire row and acquires the speed of the one that was pushed. Furthermore, there are experiments showing that all those bodies we count among the hardest, such as tempered steel, glass, and Agate, are elasticHuygens uses the term font ressort (literally "act as a spring"), referring to the property of elasticity where a material deforms under pressure and then returns to its shape., and bend in some way, not only when they are stretched into rods, but also when they are in the form of balls or otherwise. That is to say, they yield slightly inward at the spot where they are struck and immediately return to their original shape. For I have found that by striking a ball of glass or Agate against a large and very thick piece of the same material, which had a flat surface slightly dulled with breath or otherwise, round marks remained, larger or smaller depending on whether the blow had been strong or weak. This shows that these materials yield upon impact and then restore themselves, for which they must employ time.

Now, to apply this kind of movement to that which produces light, nothing prevents us from estimating that the particles...