The diagram shows the spectral absorption dips (black) associated with the hydrogen-like transitions of Wannier–Mott excitons in a copper oxide crystal. In their historic 1952 experiment4, Gross and Karryjew identified the eight dips that correspond to transition energies En, with n = 2, 3, ... 9. The transition with energy E1 is forbidden by optical-selection rules and cannot be observed. Kazimierczuk et al.5 have observed excitons in a natural crystal of copper oxide with n as large as 25. The energies are given in electronvolts.