  We perform a model-independent phenomenological analysis of experimental data
on proton form factors ratio. We find that only one of the two photon exchange
amplitudes, which we call $\delta G_M$, is responsible for the discrepancy
between Rosenbluth and polarization methods. The linearity of Rosenbluth plots
implies that $\delta G_M$ is approximately linear function of $\epsilon$. The
slope of $\delta G_M$ is extracted from experimental data and is shown to be
consistent with theoretical calculations.
