Abstract
I consider interesting to describe the conditions that prevailed at the time of the work by M. Peimbert and S. Torres-Peimbert 1971, BOTT, 6, 36, 21. For this purpose I present a discussion on the observational conditions at the time. I explain briefly some aspects of the progress that has taken place in the study of planetary nebulae. I include a description of the papers that followed this research and their contribution to our knowledge of planetary nebulae.
