Lucretius talks about the motion of dust in light, but he doesn't claim that it is the origin of the theory. When I google "Leucippus dust light" I get lots of people making my claim and more respectable sources making weaker claims, like "According to traditional accounts the philosophical idea of simulacra is linked to Leucippus’ contemplation of a ray of light that made visible airborne dust," but I don't see any citations to where this tradition is recorded.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.