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BKS theory was a 1924 attempt to describe light interacting with chemical atoms through continuous radiation fields, while allowing energy conservation and momentum conservation to hold statistically rather than in every individual atomic event. Its crucial experimental consequence was the absence of the event-by-event correlations expected from a photon collision picture of Compton scattering.

Walther Bothe and Hans Geiger tested whether a recoil electron and the photoelectron produced by the scattered radiation appeared together. Their coincident detections contradicted the predicted independent event times. In his 1925 response, John C. Slater explicitly discussed this conflict and also the observed correlation of the scattered radiation's direction with that of the recoil electron. BKS theory is therefore a useful example of testing a physical interpretation through individual-event correlations, even when average radiation intensities alone do not decide between pictures.^[ref]

John C. Slater proposed empirical atomic screening rules used to estimate an effective nuclear charge for an electron. Slater's rules assign different screening contributions according to the shells and subshells occupied by the other electrons.

The calculation approximates a many-electron atom by a simpler effective attraction. Its usefulness comes from providing a consistent estimate, rather than claiming that all atomic electrons experience one identical constant charge.^[ref]