We have compared the frequency of the hydrogen 1S-2S transition (resonance width Δν/ν=2.8×${10}^{\mathrm{-}11}$) with a reference frequency synthesized from an 88-THz infrared ${\mathrm{CH}}_4$-stabilized HeNe laser. In this first measurement of an optical frequency that overcomes the uncertainty limits of the visible ${\mathrm{I}}_2$-stabilized He-Ne laser standard, we find a 1S-2S frequency of 2 466 061 413.182(45) MHz with an 18-fold improved precision of 1.8 parts in ${10}^{11}$. A new value of the Rydberg constant, ${\mathit{R}}_{\mathrm{\infty }}$=109 737.315 684 1(42) ${\mathrm{cm}}^{\mathrm{-}1}$, is deduced.

• Received 15 July 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.1923