TREATISE

from one part of the air to another, and because the extension of this movement occurs at an equal speed in all directions, it must form something like spherical surfaces, which expand continuously and eventually strike our ear. Now, there is no doubt that light also reaches us from a luminous body to our eyes by some movement impressed upon the matter that lies between the two; for we have already seen that it cannot be by the transport of a physical body passing from one to the other. If, in addition to this, light takes time for its passage—which we are now going to examine—it will follow that this movement impressed upon the matter is successiveHuygens uses "successive" to mean that movement happens step-by-step over time, rather than happening everywhere at once (instantaneously)., and that consequently it spreads, just like that of Sound, by spherical surfaces and waves. I call them waves because of their resemblance to those one sees forming in water when a stone is thrown into it, which represent such a successive spreading in a circle, although arising from a different cause and occurring only on a flat surface.

To see then if the extension of light occurs over time, let us first consider if there are any experiments that might convince us of the contrary. As for those that can be performed here on Earth with fires placed at great distances, although they prove that light takes no sensible timeMeaning a duration of time that is perceptible to human senses or measurable by the instruments of the 1670s. to cross these distances, one can rightly say that these distances are too small, and that one can only conclude that the passage of light is extremely fast. Mr. Descartes, who was of the opinion that light is instantaneous, based his view—not without reason—on a much better experiment drawn from Lunar Eclipses; which, however, as I shall show, is not convincing. I will propose the argument slightly differently than he did, to make the full consequence better understood.

Let A be the location of the sun, and B D be a part of the orbit or path—