We can in fact first place the beam of rays of moving positive atomic ions in a plane perpendicular to the axis in which we see the spectral lines emitted by them. – Johannes Stark

Thus at the beginning of 1906 it seemed to be established that the emitters of the spectral series of chemical elements are their positive atomic ions. – Johannes Stark

By allowing the positive ions to pass through an electric field and thus giving them a certain velocity, it is possible to distinguish them from the neutral, stationary atoms. – Johannes Stark