THE MOST PRECISE FREQUENCY MEASUREMENT ever made in the visible or ultraviolet portion of the electromagnetic spectrum has been carried out at the Max Planck Institute for Quantum Optics near Munich. Measuring the frequency (or, equivalently, the energy) of a light wave is easy in the microwave region (around 10⁹ Hz), where one can directly count oscillations in an electronic circuit. This does not work for visible or ultraviolet light, so Theodor Hansch (011-49-892-180-3212) steps down UV waves by mixing them with light at lower frequencies, producing an average or "beat" signal. After numerous stages the resultant signal is amenable to high-precision counting methods (Physics Today, Dec. 1997). In this way the frequency corresponding to the important (for the study of quantum mechanics) interval between the 1S and 2S quantum states in hydrogen has been determined to be 2.466 061 413 187 34 x 10¹⁵ Hz, with an uncertainty of only 3 parts in 10¹³, an improvement by a factor of almost 100 over previous work. (Udem et al., Phys. Rev. Lett., 6 Oct.) A new article, upcoming in Phys. Rev. Lett., reports on comparable measurements for deuterium, allowing the best calculation of the difference in the mean square charge radii for the proton and the deuteron.