In order to explain better how the speed and centrifugal force of the small vortexes is the general principle of natural effects, I am going to try to prove that certain properties of light uniquely depend on it, and especially that of reflecting and undergoing refraction on, or rather, near the surface of bodies; but I shall do so after having given the demonstration I just promised, and did not produce, because some readers do not need it and most will not be able to understand it or will not wish to read it.