R["o]ntgen

from The Collaborative International Dictionary of English v.0.48

Rontgen \R["o]nt"gen\, Roentgen \Roent"gen\, a.
   Of or pertaining to the German physicist Wilhelm Konrad
   R["o]ntgen, or the rays discovered by him; as, R["o]ntgen
   apparatus.
   [Webster 1913 Suppl.]