Holden, E. S. (1893). «Monument to the Late Richard Proctor». Publications of the Astronomical Society of the Pacific5 (32): 222. Bibcode:1893PASP....5Q.222.. doi:10.1086/120721.
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Richard A. Proctor: Other worlds than ours : the plurality of worlds studied under the light of recent scientific researches. London : Longmans, Green, 1870. (Digital Copy)
Holden, E. S. (1893). «Monument to the Late Richard Proctor». Publications of the Astronomical Society of the Pacific5 (32): 222. Bibcode:1893PASP....5Q.222.. doi:10.1086/120721.