Complex sentence: Subsequent improvements in the instrument led to the development of a second and third instrument of improved mass resolving power and mass accuracy where the use of electromagnetic focusing in mass spectrograph allowed him to identify 212 naturally occurring isotopes.
Split into simpler sentences: Subsequent improvements in the instrument led to the development of a second and third instrument of improved mass resolving power and mass accuracy. These instruments employing electromagnetic focusing allowed him to identify 212 naturally occurring isotopes.