A Short History of the Subject

Abstract

Isotope shifts in optical spectra are small— they are often considered as hyperfine structure. A typical size of an isotope shift is 3 GHz or so. For visible light of wavelength 500 nm or 5000 Å (Angstrom) this is 3 × 10⁹ Hz in 6 × 10¹⁴ Hz, or in terms of wavelength, a typical shift is 2.5 pm or 0.025 Å. Such small shifts could not be observed before the advent of interferometric methods into spectroscopy. The first person to use such a method was Michelson, who with the aid of “the interferometer” was, in about 1890, deducing structures in spectral lines down to the 0.01 Å level. He was looking for a spectral line without structure whose wavelength could be used as an absolute standard of length. He proposed the cadmium red line for this (Michelson, 1893). It is probable that some of the structure in some of the lines which Michelson observed and rejected in his search for a length standard were due to shifts between isotopes. However, in view of the then current nature of atomic theory, we can hardly blame Michelson for his failure to discover isotopes via their isotope shifts.