What Have We Learned?
The electron micrographs we have shown illustrate the many improvements and advances made in spectroscopic gratings over time. Rutherfurd and Rogers made important advances, and Rowland’s engines produced first-class spectroscopic gratings well into the twentieth century — long after his death. Rowland conceived the idea of concave gratings to improve efficiency, and Anderson discovered how to make the grooves at an angle, in order to concentrate light into a specific order of a spectrum, further improving efficiency and increasing resolution.
What is most interesting is that improvements were made without the maker having any direct knowledge of what the grooves actually looked like. Even the coarsest grating tested by the CSTM is below the resolving power of light microscopes. Any early advances were made by analyzing the spectra, then deducing the causes of any flaws and errors. Ghosts of bright lines were correctly interpreted as being due to periodic errors of the ruling engine’s drive screws. This lack of direct knowledge was later overcome by Harold Babcock, who developed a method of interferometric analysis to control the diamond as it ruled the grooves. His gratings were the best available. Looking at the Bausch & Lomb gratings, one might even suggest that the tools of science took a step backwards when the Babcocks’ grating lab closed. However, the number of grooves per millimetre is double that of any of the others illustrated here, and the molecular and crystalline structure of the substrate became more significant during the ruling process.
Rowland’s early success directly influenced grating production from Anderson, to Jacomini, to Harold and Horace Babcock — from Johns Hopkins, to Mount Wilson Observatory, Cal Tech and Hale Observatories and far beyond — and covered an astonishing 75+ years. Rowland himself influenced the progress of astronomy in the U.S. as no one else has before or since, with the possible exception of George Ellery Hale. By combining engineering skill with scientific knowledge, American science was able to propel itself far ahead of its European counterpart.
