| Sumario: | The Balmer decrement was calculated for a homogeneous, spherical, pure hydrogen nebula, ionised and excited both by radiative and collisional processes. Reabsorption in the Balmer series from sublevels 2 s and 2 p was taken into account. Calculations were carried out for electron temperatures of 104 K and 2×104 K, electron densities from 103 to 106 cm-3, and radii from 1015 to 1017 cm. The results define allowed and forbidden regions in the log(Hβ/Hγ) vs log(Hα/Hβ) diagram different from those of previous results obtained assuming purely radiative or purely collisional ionisation and excitation. A brief comparison with observations is presented and discussed. © 1971 D. Reidel Publishing Company.
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