The ratio of intensities of the optically forbidden lines N(2D¿--4S¿) 519.8, 520.0 nm has been measured with high precision in the nightglow in repeated observations made with the High Resolution Echelle Spectrophotometer (HIRES) and Echellette Spectrograph and Imager (ESI) on the Keck I and Keck II telescopes, respectively, at the W.M. Keck Observatory on Mauna Kea, Hawaii. The observed intensity ratio, I(519.8 nm)/I(520.0 nm), is seen to vary little over the bulk of any particular night, with a value of 1.759 ¿ 0.014 determined over all observations. Among theoretical predictions of the ratio, this agrees best with a value of 1.788 determined from the ratio of products of recent Breit-Pauli calculation-derived spontaneous emission coefficients (Froese Fischer and Tachiev, 2004) and statistical weights of the fine structure levels of the 2D¿ term, under the assumption that these levels' populations are thermalized by rapid mixing through collisions with N2.