I should've realized back then that this era already has flashlights.
Anyway.
Now that Will has explained it, everything finally makes sense.
Why do flashlights exist, and why do I have this weird feeling? It's because...
This world's timeline isn't just a continuation after the 1665 instance.
It's been influenced by the letter I sent to Newton, on top of the 1665 instance!
Just think about it.
According to normal historical development,
It'd be 1808.
Before Marius discovered the phenomenon of light polarization.
Then in 1818, Poisson spots would officially be discovered.
The Poisson spot is pretty much the cognitive limit of classical physics in optics—at most, Faraday and Maxwell would later add their mathematical definitions, and then we'd have the photoelectric effect eighty years after that.
But now, Newton has brought forward the discovery of the Poisson spot by a full 140 years.
Even though there's a gap with Thomas Young's interference experiment in between,
