Treatise on Light

T R A I T É

[lu]miereThe word "Lumiere" (light) began on the previous page; "miere" is the conclusion of that word. will always continue with an equal speed.

And it must be understood that although the particles of the ether are not arranged in straight lines as in our row of balls, but confusedly, such that one touches several others, this does not prevent them from transporting their movement and extending it always forward. In this, there is a law of movement to be noted that serves this propagation, and which is verified by experience. This is that when a ball, such as A here, touches sev-

A scientific diagram showing a collision between spheres. A circle labeled 'B' is positioned above a group of four spheres. The central sphere of the group is labeled 'A', and it is surrounded by three identical spheres labeled 'C', all of which touch 'A'.

eral other similar ones C C C, if it is struck by another ball B, in such a way that it makes an impression on all the C C C that it touches, it transfers all its movement to them, and remains motionless afterward, as does ball B. And without assuming that the ethereal particles are spherical in shape (for I do not see any other reason why it would be necessary to assume them so), one understands well that this property of impulse nevertheless contributes to the said propagation of movement.

Equality of size seems to be more necessary there, because otherwise there must be some reflection of movement backward when it passes from a smaller particle to a larger one, according to the Rules of PercussionHuygens is referring to his own laws of elastic collision/impact, which he pioneered. He argues that if the particles were of different sizes, the "pulse" of light would bounce back rather than moving cleanly forward. that I published some years ago.

However, we shall see hereafter that we do not need to assume this equality for the propagation of light so much as to make it easier and stronger; it being also not unlikely that the particles of the ether were made equal for such a significant effect as that of light, at least in that vast expanse which is beyond the original: "region des vapeurs"region of vaporsThe Earth's atmosphere. Huygens distinguishes between the messy, mixed air near Earth and the "pure" ether of deep space., which seems only to serve to trans-