On today's date in 1731 was born Henry Cavendish. He is mainly know for his Chemistry research, however, while I was reading his biography I've noticed that he also calculated the bending of light due to the Sun - see e.g. NASA's summary on the topic.
Apparently he used Newtonian physics and his results weren't so far from the actual. My question is how did he do his calculations? I thought Newtonian gravity was postulated to act only on massive particles... Did he assume light is massive or what?
Answer
First, the page you linked to only says that Cavendish calculated the deflection of light, not measured it. Newton's law of gravity technically only applies to massive particles, as you say. But this business about light being massless came a hundred years later than Cavendish; in his time light was regarded as a wave, though Newton liked the idea of particles of light and called them "corpuscles".
The important point, though, is that since Galileo people had been aware that the acceleration due to gravity is independent of the object's mass. So it wouldn't have been a stretch to assume that whatever light is, it would be affected by gravity in exactly the same way as everything else, and Cavendish made his calculation based on this assumption.
The idea of light being bent by gravity therefore predates Einstein. General Relativity provides a more solid basis for this idea, but in practice the most important point is that the deflection it predicts is twice as large as the classical one. When Eddington measured the shift of a star during a solar eclipse, he wasn't only checking that light was deflected, he was also checking the actual formula predicted by GR.
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