> There was a debate about whether light was a wave or a particle -- evidence supported both views.

Not before the 20th century,

Yes before the 20th century. I refer you to the debate between Newton and Huygens as well as Hooke, his contemporaries with respect to these ideas.

http://www.studyphysics.ca/newnotes/20/unit04_light/chp1719_...

Such debates persisted from then until the shaping of quantum theory.