"There was a way to make individual alpha particles visible using zinc sulfide... A small glass plate coated with zinc sulfide and bombarded with alpha particles briefly fluoresced at the point where each particle struck, a phenomenon known as "scintillation" from the Greek word for spark. Under a microscope the faint scintillations in the zinc sulfide could be individually distinguished and counted. The method was tedious in the extreme. It required sitting for at least thirty minutes in a dark room to adapt the eyes, then taking counting turns of only a minute at a time -- the change signaled by a timer that rang a bell -- because focusing the eyes consistently on a small, dim screen was impossible for much longer than that. Even through the microscope the scintillations hovered at the edge of visibility; a counter who expected the experiment to produce a certain number of scintillations sometimes unintentionally saw imaginary flashes."
Thank god they invented the photomultiplier before I arrived on the scene.