By direct measurement of the contours of the line wings, the resonance broadening of the sodium $D$ lines in absorption was determined for vapor pressures of sodium ranging from ${10}^{-3\mathrm{}}$ to 70 mm Hg with negligible amount of foreign gases. Over nearly the entire range, the half-intensity width varied linearly with the density of the absorbing atoms; i.e., $\frac{\Delta {\nu }_{\frac{1}{2}}}{N}=0.80\mathrm{}\times {10}^{-7\mathrm{}}$ ${\sec .\mathrm{}}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-3\mathrm{}}$—the constant being 1.6 times that predicted by theory but much less than the corresponding experimental results for potassium. Above 5 mm pressure, $\Delta {\nu }_{\frac{1}{2}}$ increased slightly faster than $N$; moreover, the contour for the absorption coefficient was more adequately represented by an inverse cube contour than by the inverse square. Somewhat in contradiction to theory, the relative width, $\frac{\Delta {\nu }_{\frac{1}{2},8\mathrm{}}}{\Delta {\nu }_{\frac{1}{2},l}}$ was 1.12. No definite evidence of van der Waals force was observed at the higher pressures; likewise no appreciable shifts or asymmetries. The natural width measured was consistent with previous results.

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