Thomas Young made the first breakthrough deciphering the Rosetta Stone's hieroglyphs, demonstrated that light was a wave**, and realised that eyes must contain different receptors for registering blue, green and red light.
This information from We Need to Talk About Kelvin, whose author, Marcus Chown, is guesting with Matt Beaumont at this month's Firestation Bookswap, where I am a stand-in host.
* It actually took him longer than a day. Loser
** For now, let's say it is 'wavelike' and leave it at that