Cachet depicts a light beam passing through a prism to create a spectrum. Stamp depicts colors of the spectrum. The 15th Colloquium Spectroscopicum Internationale was held May 26-30, 1969 in Madrid, Spain.
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Fábrica Nacional de Moneda y Timbre (Spain), and S.F.C. “First Day Cover Commemorating the XV Colloquium Spectroscopicum Internationale.” Spain, 1969. Witco Stamp Collection, Box 5. Science History Institute. Philadelphia. https://digital.sciencehistory.org/works/08612n619.
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