They assumed that Isaac Newton’s theory of gravity was the full picture, and that light behaves like any other particle in response to Newtonian gravity.

As the 19th century came to a close, physicists were feeling pretty satisfied with the state of their science. The great edifice of physical theory seemed complete. A few minor experiments ...

For a long time, scientists believed that light traveled as particles, which is what Isaac Newton believed, and he seems to be correct. Other scientists such as Christiaan Huygens, however ...

The wave-theory itself cannot be correct, but except for its greater age it has no greater claims than the light-quantum view.