The Continuous Spectra of Certain Planetary Nebulæ A Photometric Study. T. L. Page in Mon. Notices Royal Astron. Soc. 96:604 (1936).  What the paper says?
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This is the paper that first observed continuous-spectra from planetary nebulae in the visible range (first continuous were by Slipher[1]) but it failed to understand the mechanism.

References

  1. On the spectrum of the nebula in the Pleiades. V. M. Slipher in Low. Obs. Bull. 2:26 (1912).