Let's briefly recapitulate the situation in 1865: MAXWELL'S EQUATIONS, which correctly described all the phenomena of electromagnetism known in the mid-19 Century (and then some), predicted also that electromagnetic fields should satisfy the WAVE EQUATION - i.e., by virtue of a changing creating and vice versa, the electric and magnetic fields would be able to "play off each other" and propagate through space in the form of a wave with all the properties of light (or its manifestations in shorter and longer wavelengths, which we also term "light" when discussing electromagnetic waves in general). Fine, so far.
But there are some unsettling implications of this "final" explanation of light. First of all (and the focus of this Chapter) is the omission of any reference to a medium that does the "wiggling" as the electromagnetic wave goes through it. Water waves propagate through water, sound waves through air, liquid or solid, plasma waves through plasmas, etc. This was the first time anyone had ever postulated a wave that just propagated by itself through empty vacuum (or "free space," as it is often called in this context). Moreover, the propagation velocity of light (or any electromagnetic wave) through the vacuum is given unambiguously by MAXWELL'S EQUATIONS to be m/s, regardless of the motion of the observer.