Direct observation of the transversal Doppler-shift

An experiment is reported in which the second order Doppler-shift has been determined by the observation of the Hα-line emitted by linearly moving hydrogen atoms of velocities 2.53×10⁸ cm/s-9.28×10⁸ cm/s. In contrast to previous experiments the direct transversal observation has been used for the first time. The coefficient of the second order term in the relativistic approximation is found to be 0.52±0.03 which compares good with the theoretical value of 1/2.