Intervals between the 10G, H, I, K, and L (L=4--8) states of helium have been measured with high precision using a fast-beam microwave-laser technique. The uncertainties in the measured intervals are less than 1 kHz (or about 10^-13 a.u.) for each of the 16 intervals. The measurements are compared with precise theoretical predictions and are used to test relativistic, QED, and retardation effects in these states. The measurements confirm the contributions from ``retardation forces'' to better than 10%.
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Hessels et al. (1992) studied this question.
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