@article{11524,
  author = {Lindsey Smith and Lawrence Aller},
  title = {On the Classification of Emission-Line Spectra of Planetary Nuclei},
  year = {1969},
  journal = {The Astrophysical Journal},
  volume = {157},
  pages = {1245},
  month = {Jan-08-1969},
  issn = {0004-637X},
  url = {http://adsabs.harvard.edu/full/1969ApJ...157.1245S},
  doi = {10.1086/150151},
}