I haven't felt like blogging anything in a while, but I am currently reading Newton's Opticks. I had always heard that Newton held back progress in optics by promoting a ray theory, but I am struck by the clarity of his thought and care of his experiments. He really made a lot of progress on color.
Furthermore, in Book II, Part II, Proposition XII, he acknowledges that light has an oscillatory behavior. He knew that these rays had some characteristic that oscillated in time. This part has echoes of "hypotheses non fingo", his refusal to pronounce on the nature of gravity:
"What kind of action or disposition this is; Whether it consists in a circulating or vibrating motion of the Ray, or of the Medium, or something else, I do not here enquire."
He goes on to concede that a wave model might work, but says he will not pursue that.