> the rationale that makes them confident that this is only a single atom Nothing photographic, as I understand it. The atom is positively charged, and is susp

> the rationale that makes them confident that this is only a single atom Nothing photographic, as I understand it. The atom is positively charged, and is suspended in ultra-high vacuum by electric fields. Effectively they're "weighing" it, by measuring the total charge of whatever is suspended, and knowing (from the ionization energy?) the charge on one atom. It should be noted that this is similar to how Millikan first measured the electron charge back in 1909.