Complex sentence: Chladni repeated the pioneering experiments of Robert Hooke of Oxford University who, on July 8,1680, had observed the nodal patterns associated with the vibrations of glass plates.
Split into simpler sentences: Chladni repeated the pioneering experiments of Robert Hooke of Oxford University. On July 8,1680, Hooke had been able to see the nodal patterns associated with the modes of vibration of glass plates.