"We have thus found that there are several different kinds of work to be done by the electro-magnetic medium if it exists. We have also seem that magnetism has an intimate relation to light, and we know that there is a theory of light which supposes it to consist of the vibrations of a medium. How is this luminiferous medium related to our electro-magnetic medium?"
January 1, 1970
https://en.wikiquote.org/wiki/On_Action_at_a_Distance