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A 633-nm He-Ne laser has been stabilized on a hyperfine component of an absorption line of natural iodine vapor by means of a servo system utilizing the zero crossing of the third harmonic of an applied modulation frequency. In its present form the laser has a 1000-sec stability of 1 x 10(-11) lambda, and by virtue of the great reduction in background offset problems resulting from the third harmonic servo, a reproducibility without manual interference of 2 x 10(-10) lambda; improvement in both of these characteristics appears straightforward. The absolute wavelength measured against the Kr(86) standard is 632, 991.398 +/- 0.003 pm.

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Direct-frequency measurements have been extended into the visible region of the electromagnetic spectrum. The visible frequencies were synthesized by generating the second harmonic of the recently measured 260-THz (20)Ne, 1.15-microm laser with a LiNbO(3) crystal. The absolute frequencies of ten hyperfine components of (127)I(2) near 520 THz are reported.