@article{rosenthal1971ultra,
  author = {Rosenthal, Earl},
  title = {Plus ultra, non plus ultra, and the Columnar Device of Emperor Charles V},
  journal = {Journal of the Warburg and Courtauld Institutes},
  year = {1971},
  volume = {34},
  pages = {204-228}
}

@article{rosenthal1973invention,
  author = {Rosenthal, Earl},
  title = {The Invention of the Columnar Device of Emperor Charles V at the Court of Burgundy in Flanders in 1516},
  journal = {Journal of the Warburg and Courtauld Institutes},
  year = {1973},
  volume = {36},
  pages = {198-230}
}