“Ionization Structure of Gaseous Nebulae: Sulphur, Nitrogen and Helium”, RMxAA, vol. 1, no. 1, p. 129, Jan. 1974, Accessed: Dec. 11, 2025. [Online]. Available: https://astronomia.unam.mx/journals/rmxaa/article/view/1974rmxaa...1..129p