  The nonresonant (NR) corrections are estimated for the most accurately
measured two-photon transition 1s-2s in the hydrogen atom. These corrections
depend on the measurement process and set a limit for the accuracy of atomic
frequency measurements. With the measurement process adopted in the modern
experiments the NR contribution for 1s-2s transition energy can reach 10^{-3}
Hz while the experimental inaccuracy is quoted to be +/- 46 Hz.
