This thesis has involved an attempt to determine the ratio of electric-quadrupole radiation to magnetic-dipole radiation for the forbidden transition 6647A(2D5/2-2D3/2) of ionized mercury. The high resolution needed for this type of work was obtained by using a 14mm Fabry-Perot etalon and a three prism glass spectrograph, The mercury source was a heated eleotrodeless quartz discharge tube excited by an rf oscillator. A large electromagnet was used to obtain the Zeeman effect. This is the first time that the 6647A transition has been analyzed. The experimental results for the longitudinal view show agreement with predictions for the Zeeman components giving complete verification of the identity of the line but the intensity patterns show a possible disagreement with theoretical predictions for the ratio of magnetic-dipole to electric-quadrupole radiation.