  This paper has been temporarily withdrawn by the authors. In the process of
4He 2P fine structure measurement in a vapor cell, highly nonlinear dependences
of 2S_1 - 2P_J intervals on helium density are observed and attributed to light
pressure. If total angular momentum of the excited state is not greater than
that of the ground state, there are large polarization dependences and
variations of shifts with helium pressure that exhibit apparent singularities.
A pressure-variable cell may be a better system for control of this effect than
an atomic beam. A method of avoiding the shifts for the fine structure
measurement is suggested.
