I should have realized at the time that flashlights existed in this era.
In summary.
Now that Will explained it, everything became clear.
Why flashlights exist, why I felt a sense of dissonance, it's because...
The timeline of this world is not the one inferred after the 1665 Dungeon.
But a world that was also influenced by the letter I previously sent to Little Niu on the basis of the 1665 Dungeon!
Think about it.
According to normal historical development,
it would be 1808.
Marius would only discover the phenomenon of light polarization then.
Subsequently, in 1818, Poisson's spot would officially be discovered.
Poisson's spot can basically be considered the cognitive limit of Classical Physics in optics—add some mathematical definitions from Faraday and Maxwell at most, and then it's the photoelectric effect 80 years later.
As for Little Niu now, he advanced the discovery time of Poisson's spot by a full 140 years.
