Rowland's gratings: contemporary technology

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Henry A. Rowland, whose contributions to spectroscopy we celebrate today, was an engineer turned physicist who, early in his career, showed little interest in optics in general or spectroscopy in particular.(l) By the 187Os, however, diffraction gratings were hot topic in the physics community. Following the insights of Bunsen and Kirchoff, published in 1859, scientists in Europe and the United States rushed to observe solar, stellar, and planetary spectra, to map their lines, and to correlate absorption lines with emission lines observed in the laboratory. Most of this early work was done with prisms. Diffraction spectra had been observed repeatedly during the previous 75 years but, owing to lack of both conceptual framework and instrumental means, interest could not be sustained. David Rittenhouse, the leading scientist in revolutionary America, had observed the spectrum formed by grating composed of parallel wires in 1785. Fraunhofer likewise constructed grating of 260 parallel wires.(2) Wire gratings were obviously inferior to those ruled on metal or glass. But, as John Herschel noted, construction of gratings proper for these delicate purposes...was no easy matter. (3) Each improvement in the technique, therefore, was introduced with pride and greeted with enthusiasm. Since parallel lines scratched onto hard surface could serve several purposes--such as printing plates for paper currency--machines for creating them have long history.(l) The first ruling engine used at least in part for diffraction gratings belonged to John Barton, Comptroller of the Mint in London. Barton was related by marriage to John Harrison, the celebrated clock maker whose chronometer had solved the longitude problem. Equally important--since the screw is the key element in ruling engine--Barton was friend of Henry Maudslay, the British engineer famous for improving and standardizing the screw. In 1810 Barton and Maudslay each made screw of the same pitch, and 15 inches long, and the two when placed side by side were found exactly to agree throughout their length, and were considered perfect. The engine on which Barton ruled his gratings was one he had inherited from Harrison, improved, I suspect, with one of his own screws. It is now in the Science Museum in London.(5) main purpose in activating the engine was to produce Iris Ornaments--named after the Greek goddess of the rainbow--and to that end he patented, in 1822, a certain Process for the application of Prismatic Colours to the Surface of Steel and other Metals, and using the same in the manufacture of various Ornaments.(6) The scientific utility of Barton's Buttons was