[Bored chemist]

Granted that ε and μ are frequency-dependent in a medium, as noted by Newton*, but that still doesn't address Halc's assertion that the propagation speed of light depends on the speed of the medium.

*Another mystery. Newton bought his prism from a stall in Cambridge market (the market still exists and is reputedly the oldest 7-day charter market in Europe). If nobody had noticed refractive dispersal before 1666, why was anyone making and selling glass prisms in a general market?   

People had noticed it.
They even made toy prisms to show it.
Newton Studied it.

Not sure you have grasped the right end of the stick there, BC.

You forgot to say what stick you were talking about.

The idea of a refractive index can be thought of by saying that light travels at C, but, only when it's between molecules.
When it hits one, it is trapped (very briefly) then released.

[Colin2B]

I think Halc means the Fizeau experiment with light through moving liquids.

[alancalverd]

Aha! I am enlightened!

[Colin2B]

Aha! I am enlightened!

So, you’ve seen the light, hopefully at constant speed!

[hamdani yusuf]

- As it travels through the extremely good vacuum of intergalactic space, the low frequencies travel slightly slower than the high frequencies

Afaik, the low frequencies are affected less by dielectric media than high frequency light. Blue light is deflected further by prism than red light.

[hamdani yusuf]

People had noticed it.
They even made toy prisms to show it.
Newton Studied it.

Similar thing happened with magnet. People made many interesting things without understanding how they work.

[hamdani yusuf]

- So sunlight passing through water droplets in the air are diffracted by (slightly) different angles
- This separates out sunlight into its constituent rainbow of colors

In the case of rainbow, it is refraction instead of diffraction which produces color separation.