• A beam (the alpha particles)
• A target (the gold atoms in the foil)
• A detector (the zinc sulfide screen)

No one can actually "see" atoms or sub-atomic particles because they are just too small. However, physicists like Rutherford can shoot particle beams at small things like atoms, and record what happens to these beams using a detector. Armed with this knowledge, the physicists can determine the nature of particles and interactions.