In the 1870s he began an ambitious survey to amass stellar spectra, from which he hoped to establish a spectral classification that would reveal something about stellar evolution. In this objective he failed, but in 1888 he detected Doppler shifts in stellar spectra which enabled him to measure reliable radial velocities for the first time. The next year he discovered that Algol and Spica were spectroscopic binary stars, and derived their masses and orbits.