The optical radiation from plasma discharges of electron-bombardment mercury-ion thrusters was investigated. Spectrographic measurements indicated that the discharge was composed primarily of mercury atoms and singly charged ions. Excitation spectra of doubly charged mercury ions was measured to obtain the fraction of such ions in the discharge. Accomplishments of spectroscopic measurements of a hollow cathode thruster included the identification of two diagnostic lines in the mercury spectrum and the interpretation of the spectral amplitudes in terms of a superposition of primary and Maxwellian electron distributions. Potential application of optical techniques to thruster control applications was also suggested by the measurements.Include...