The absolute scale of the Hg 6¹P₁ direct electron impact excitation cross section has been determined in the positive column of low-pressure mercury discharge. The discharge was studied spectroscopically in a regime where direct excitation and radiative decay are the dominant population and depopulation mechanisms of levels. Using a dye-laser absorption method the author determined the number density of the Hg 6¹P₁ level in the steady state as well as its decay rate in the afterglow under various discharge conditions. From the data the author deduces the excitation rates of the 6¹P₁ level. The electron densities and temperatures in the tail of the energy distribution were obtained from measurements of the radiances at optically thin lines with known excitation functions. With the shape of the excitation function of the 184.9 nm line given in the literature, its absolute scale is obtained from the data. The cascade contribution to this excitation function is less than 15% for 6.7<or=E_e<or=15 eV. The initial slope of the 6¹P₁ direct excitation cross section is (0.35+or-0.1)*10^-20 m² eV^-1, the cross section at 15 eV is (2.1+or-0.6)*10^-20 m².