= Michelson measurement of length in cadmium wavelengths
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<Albert Abraham Michelson>'s 1907 lecture, https://www.nobelprize.org/uploads/2018/06/michelson-lecture.pdf[Recent Advances in Spectroscopy], pp. 171–172, connects counting <interference fringes> with a reproducible length standard. Broad or multiple <spectral lines> obscured fringes at large path differences; red <cadmium> emission supplied a suitable source. His 1907 <Nobel Prize in Physics> recognized optical precision instruments and their <spectroscopy> and <metrology> applications.<Nobel Lectures, Physics 1901–1921/Recent Advances in Spectroscopy by Albert Abraham Michelson>[Nobel Lectures, Physics 1901–1921; Michelson, Nobel lecture, printed pp. 171–172; official prize motivation, printed p. 158]{ref}
His authored account of the metre comparison at Sèvres describes a heated, evacuated <cadmium> discharge tube and a <spectrometer> selecting one <spectral line>. A movable reference mirror was compared with pairs of silvered parallel surfaces defining intermediate standards. Counted <interference fringes> established a short interval; repeated comparisons extended it to a decimetre and then a metre. White-light fringes located surface coincidence, while monochromatic fringes supplied counts and fractional orders. A thermally shielding box, firm pier, calibrated translation screw and tested <thermometers> controlled drafts, vibration, travel and <temperature>. Uniform <temperature> mattered in comparisons; the final standard required an absolute <temperature> reading. <Optical microscopes> made the final comparison to the marked metre bar, limiting the whole result.<Light Waves and Their Uses by Albert Abraham Michelson/Lecture V: Light Waves as Standards of Length>[Michelson, Light Waves and Their Uses (1903), Lecture V, Figs. 70–77: intermediate standards, counting, temperature controls and final microscope comparison]{ref}
This is a measurement chain, rather than one enormous unchecked fringe count: short intervals establish whole orders; fractional-order readings refine each longer comparison. The <optical coherence> envelope helps locate white-light coincidence, whereas the carrier <phase difference> measures small departures from it. Repetition checks counting mistakes, but agreement alone cannot exclude shared <systematic errors>. The general lesson of <metrology> is to preserve the link from the optical ruler through each transfer and control to the final length.
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