A Google search for the terms "isaac newton lecture empty room" will return a litany of sites stating the claim that while Newton was a professor at Cambridge, he would often have nobody show up to his lectures, and he would give the lecture to the empty hall anyway, rather than doing something more profitable with his time.
Newton was undeniably a very eccentric individual, and yet the idea that he would blatantly waste his own time in this manner is just a bit too much for me. It seems more likely to me that this rather sensational claim was extrapolated from some more benign piece of trivia about his time as a professor at Cambridge.
What evidence is there to support the idea that any of these lectures to empty rooms took place?