Wave-number data.—The blue-green absorption bands of lithium have been photographed in the second order of a 21-foot grating. In the main, ${\mathrm{Li}}^7$${\mathrm{Li}}^7$, system the 2,0, 1,0, 0,0, 0,1 and 0,2 bands and in the isotopic, ${\mathrm{Li}}^6$${\mathrm{Li}}^7$, system the 1,0, 0,0, 0,1 bands and a somewhat incomplete 0,2 band have been measured and analyzed. Wave numbers of the lines of all these bands are tabulated. An unsuccessful attempt was made to identify ${\mathrm{Li}}^6$${\mathrm{Li}}^6$ band lines.

Vibrational constants.—Values of $\Delta G_v$ are calculated by least squares and from them are deduced the vibrational constants of the main and isotopic systems. The constants are given by the following equations: $\Delta {G_v}^{\prime \prime }=351.374-5.181\mathrm{} (v^{\prime \prime }+\frac{1}{2})$, $\Delta {G_v}^{\prime }=270.941-6.266\mathrm{} (v^{\prime }+\frac{1}{2})$, $\Delta {{G_v}^{\prime \prime }}^i=365.923-5.619\mathrm{} (v^{\prime \prime }+\frac{1}{2})$, $\Delta {{G_v}^{\prime }}^i=282.081-6.791\mathrm{} (v^{\prime }+\frac{1}{2})$. The isotopic mass coefficient $\rho ={\left(\frac{\mu }{{\mu }^i}\right)}^{\frac{1}{2}}=\frac{{{\omega }_e}^i}{{\omega }_e}$ was calculated for the lower and upper states. The resulting values are, from the ${}_{}{}^1{}_{}{}^{}\Sigma$ state, $\rho =1.04141\mathrm{}\pm 0.00008$ (considered the most trustworthy figure) and from the ${}_{}{}^1{}_{}{}^{}\Pi$ state, $\rho =1.0411\mathrm{}\pm 0.0002$. These results are shown to correspond to a higher isotopic mass ratio ${\mathrm{Li}}^7$/${\mathrm{Li}}^6$ than that indicated by the massspectrograph results of Costa. By employing the $Q$ branch of the 0,2 isotope band, $\Delta {{G_{1\frac{1}{2}}}^{\prime \prime }}^i$ was found and the relation $\frac{{x_e}^i}{x_e}=\rho$ was verified to within the probable error.

Rotational and electronic constants.—From rotational term differences, values of $B_v$ were computed for the main and isotopic systems by least squares. The calculated constants are: ${B_v}^{\prime \prime }=0.6721-0.00708\mathrm{} (v^{\prime \prime }+\frac{1}{2})$, ${B_v}^{\prime }=0.5577-0.00888\mathrm{} (v^{\prime }+\frac{1}{2})$, ${{B_v}^{\prime \prime }}^i=0.7302-0.00830\mathrm{} (v^{\prime \prime }+\frac{1}{2})$, ${{B_v}^{\prime }}^i=0.6046-0.00922\mathrm{} (v^{\prime }+\frac{1}{2})$. From the relation $\frac{{B_e}^i}{B_e}={\rho }^2$ are obtained values of $\rho$ which agree to within their probable error (one part in 1500) with the more accurate results obtained from the vibrational constants. The $\Lambda$-doubling for the two lowest vibrational levels of the ${}_{}{}^1{}_{}{}^{}\Pi$ state was investigated and found sensibly equal for the main and isotopic band systems. The origins of the two band systems were computed to be ${\nu }_e=20436.25\mathrm{}\pm 0.02$ and ${{\nu }_e}^i=20436.29\mathrm{}\pm 0.04$, indicating no measurable electronic isotope effect.

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