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-spectra from reflection nebulae were by Slipher) and as due to the two-photon mechanism, although the origin remained uncertain until the work of Spitzer and Greenstein~.