Well, I left out the next sentence of the book, in which Geiger builds an electronic counter (perhaps you've heard of it? :) and uses it to cross-check the humans.

According to the book, after using the counter to confirm that properly trained and rested humans were as accurate as the electronics, the next step was to put the electronic counter away because the humans could also give you spatial information: They could not only tell you that a flash had happened, but could tell you where it was in the visual field. Multichannel PMTs or (god bless 'em) chilled semiconductor-based multi-element low-noise photodetectors were still dozens of years in the future...