Precise experimental test of calculated two-electron lamb shifts in helium

We have measured the wave number of the {sup 4}He 2{sup 1}{ital S}-3{sup 1}{ital P} transition by Doppler-free spectroscopy in a metastable helium beam. The result, 19 931.924 794(45) cm{sup {minus}1}, represents an order-of-magnitude improvement over previous measurements and provides a sensitive test of two-electron QED effects. Our result shows an unexpectedly large deviation from theoretical prediction. We derive improved term values for a number of low-lying helium levels. All levels with {ital n}=2 show significant deviations from theory that are correlated with the size of calculted two-electron contributions to the Lamb shifts.