1. New York, Macmillan (1961)
2. Fourth edition, Macmillan (1977), ISBN 00-236-0190-6.
3. See American Institute of Physics Website:
www.aip.org/history/cosmology/tools/pic-spectroscopy-orion.htm
.
4. In Mapping the Sky: Past Heritage and Future Directions, IAU Symposium 133, Paris, 143–148 (1988). [1988IAUS..133.143M]
5. The European Physical Journal: Historical Perspectives on Contemporary Physics. [2012arXiv1209.3563P]