In the ytterbium test, atoms were excited by laser light in the presence of electric and magnetic fields, and the resulting parity violation was observed. The specific transition observed was the forbidden transition from 6s S 0 to 5d6s D 1 (24489 cm). The latter state was mixed, due to weak interaction, with 6s6p P 1 (25068 cm) to a degree proportional to the nuclear weak charge.