photographic representation of probability of electron densities in
various “orbitals” (energy eigenstates), from Robert Leighton, Principles
of Modern Physics (MGraw-Hill, 1959, p. 179), an especially fine copy
donated recently to the Niels Bohr Library by Martin Klein. Leighton
was not the first to make such a representation but his widely-used
textbook exposed it to millions of physicists.