The suggestion by Lewis1 in a recent Letter to the Editor advocating the use of frequency with terahertz as the basis unit for visible light instead of wavelength with nanometer is a well founded though very old one. Over the years, such a call has appeared from time to time and in various guises, particularly when it has come to the representation of spectra. According to Lord Rayleigh,2 the Anglo-Irish physicist George Johnstone Stoney (1826–1911) was the first to propose and advance the use of frequency over wavelength along the abscissa for visible light spectra. What was his reason for advocating the use of frequency? He, as many others have subsequently realized, saw frequency as the more fundamental quantity for light because it remains unchanged on passing from one medium to another. Shurcliff,3 writing some 60 years after Stoney’s initial proposal on the question concerning the most...

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