The First “Scientific Quality” Gratings

The quality of gratings depends primarily on a high-precision screw which must be perfectly regular, straight and free of periodic errors. Seeing the limitations of Rutherfurd’s gratings — i.e., ghost images of lines in spectra from periodic errors in the ruling engine’s lead screw — Harvard University’s William Rogers tried to make a perfect screw (his were probably the best-made in the nineteenth century) for a new ruling engine. However, physicist Henry Rowland at The Johns Hopkins University soon made Rogers’s efforts superfluous.

Henry Rowland (1848–1901) was an engineer, and his instrument-maker, Theodore Schneider, had access to high-quality machine tools. Rowland recognized that they were unlikely to build a perfect apparatus, but was clever enough to design apparatus that overcame several flaws, reducing both eccentricity and periodic errors of the screw, and friction and wear in the moving parts. Using superior optical blanks from John Brashear of Pittsburgh, Rowland’s gratings received wide acclaim. Rowland’s first engine could rule areas of approximately 15 x 10 centimetres (6 x 4 inches), with up to 107,500 lines to the centimetre (43,000 lines to the inch) and as many as 400,000 (160,000) lines in total. According to Rowland, the lead screw was nearly perfect; he was unable to detect any errors to .000025 millimetre (1/100,000th inch). By 1901, some 250–300 gratings had been sold and more given away. Rowland’s most important achievement, made in 1882, was recognizing that concave gratings would improve efficiency tremendously.

Henry Rowland (1848–1901) with his ruling engine (Courtesy The Johns Hopkins University, Special Collections)

Rowland’s last engine, damaged by fire before his death, was rebuilt, with improvements, by John Anderson, around 1910. He began making “blazed” gratings which focus light into a particular spectrum order of a spectrum by using sloped sides in the grooves. The key was using diamonds with a curved edge; these diamonds were also more durable and more predictable than the sharp, pointed diamonds previously used. Anderson, and Italian instrument-maker Clement Jacomini, constructed the Mount Wilson Observatory’s “A” ruling machine (1912–1915). A second “B” machine was constructed in 1933 by Jacomini, and was used with modifications (including interferometric control). In the 1940s and 1950s, Harold Babcock and his son Horace made the ultimate in grating engines, but with the closure of the Mount Wilson lab in 1963, scientists had to turn to Bausch & Lomb, Fisher Scientific and Diffraction Products Inc. for gratings. In recent years, gratings have been made using laser interferometry techniques at greatly reduced cost and superior performance.

Clement Jacomini with the Mount Wilson Observatory’s “A” ruling engine (Mount Wilson and Las Campanas Observatories of the Carnegie Institutions of Washington)