Treatise on Light

OF LIGHT. CHAP. I.

...production, one finds that here on Earth it is primarily fire and flame that create it, which undoubtedly contain bodies in rapid motion, since they dissolve and melt many other very solid bodies. Whether one observes its effects, one sees that when light is gathered together, as by concave mirrors, it has the power to burn just like fire—that is, it disunites the parts of bodies In the 17th century, "disuniting the parts" meant breaking the physical bonds between the tiny particles that make up a solid object.. This certainly indicates movement, at least in true Philosophy, in which the cause of all natural effects is conceived through mechanical reasoning. In my opinion, one must do this, or else give up all hope of ever understanding anything in Physics.

And as, following this Philosophy, it is held for certain that the sensation of sight is only stimulated by the impression of some movement of a matter that acts upon the nerves at the back of our eyes, this is yet another reason to believe that light consists of a movement of the matter found between us and the luminous body.

Furthermore, when one considers the extreme speed with which light spreads in all directions, and that when it comes from different places—even those completely opposite—the rays pass through one another without hindering each other; one understands well that when we see a luminous object, it cannot be by the transport of a matter which travels from that object to us, as a ball or an arrow crosses the air. For certainly, that contradicts those two qualities of light too much, and especially the latter. It is therefore in another manner that it spreads; and what can lead us to understand this is the knowledge we have of the spreading of SoundHuygens is famously introducing the "wave theory" of light here, using sound waves as an analogy for how light might travel through a medium. in the air.

We know that by means of the air, which is an invisible and impalpable body, Sound spreads all around the place where it was produced, by a movement which passes successively...