sentences:  In 1922, he became director of Physical Laboratory ("Fysisch Laboratorium") and extended his research interests to experimental subjects.His precision measurements concerning intensities of spectral lines brought Physical Laboratory in the international limelight.He is also remembered for the Ornstein-Zernike theory (named after Ornstein and Frederik Zernike) concerning correlation functions,
  and the Ornstein-Uhlenbeck process (named after Ornstein and George Uhlenbeck), a stochastic process.