The atom can't be seen, yet its existence can be proved. And it is simple to prove that it can't ever be seen. It has to be studied by indirect evidence — and the technical difficulty has been compared to asking a man who has never seen a piano to describe a piano from the sound it would make falling downstairs in the dark.

As quoted in Carl Anderson. Some notes about his life and work at Caltech. The first of a series of biographical sketches of Caltech faculty members. Engineering and Science, Vol. 15:1 (October 1951)

Engineering and Science, Volume 15:1, October 1951[calteches.library.caltech.edu]