Doppler suggested that it would be possible to determine
the radial component of a star's velocity by observing the shift in its
color. For double star systems the star approaching us should be shifted
to the blue whereas the star receding from us should be shifted to the
red. The typical velocities involved would not allow one to detect this
by the naked eye. It required the discovery of discrete stellar spectral
lines and their corresponding earthly counterparts to observe this effect.