Two-Electron, One-Photon Transition Energies

Wölfli's experiment about $K^{-2\mathrm{}}\to L^{-2\mathrm{}}$ two-electron, one-photon transitions was criticized by Nagel et al. in a recent paper. In the present Comment I discuss arguments of Nagel et al. and show that Wölfli's interpretation about cooperative x-ray transition is valid.