A completely electronic sweep spectroscope of high resolution has been developed for the wave-length region from 2 to 3 millimeters (150,000 mc/sec. to 100,000 mc/sec.). The radiation source is a silicon crystal harmonic generator driven by Ratheon klystrons. Silicon crystals are used also as detectors. Radiation of wave-length 1.96 mm (154,000 mc/sec.) has been generated and detected with strength more than ten times the noise level. Some lines in the hyperfine structure of the $J=19\mathrm{}\to 20$ rotational transition of ICN at 2.32 mm have been observed. The $J=1\mathrm{}\to 2$ transition of C${\mathrm{H}}_3$F has been observed at 2.93, and the separation of the $K=1\mathrm{}$ and $K=0\mathrm{}$ lines has been determined as 1.8 mc. Precision measurements on the first rotational line ($J=0\mathrm{}\to 1$) of ${\mathrm{C}}^{12}$${\mathrm{O}}^{16}$, and ${\mathrm{C}}^{13}$${\mathrm{O}}^{16}$ in the 2.6-mm region have been made with an electronic frequency standard monitored by the 10 mc/sec. signal of station WWV. The following molecular constants were determined: for ${\mathrm{C}}^{12}$${\mathrm{O}}^{16}$, $B_0={1.92262}_5$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B_e={1.93136}_7$ ${\mathrm{cm}}^{-1\mathrm{}}$, $I_0={14.5553}_1\times {10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$, $I_e={14.4894}_3\times {10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$, $r_0={1.13078}_{9}A$, $r_e={1.12822}_{7}A$; for ${\mathrm{C}}^{13}$${\mathrm{O}}^{16}$ $B_0={1.83806}_9$ ${\mathrm{cm}}^{-1\mathrm{}}$, $I_0={15.2248}_9\times {10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$, $r_0={1.13072}_{5}A$. The last three digits quoted for $r$ and $I$ have relative significance only, because of possible error in Planck's constant.

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