Lynn Poole describes a diffraction grating as having ridged scales, like a butterfly's wings, that create iridescence. The 75,000 precise ridges must be so accurately spaced that it requires a special machine to make a diffraction grating. Dr. Henry H. Rowland developed the first one in 1860, but Dr. John Strong refined this machine, the ruling engine, in 1950. Using a model of the machine, Dr. Strong explains the operation of a ruling engine, focusing on the perfect twin screws that are its vital parts. The maker of these screws, David Broadhead, of Wellsville, NY, demonstrates how one cuts one of these screws on a machine lathe. In a film, Wilbur Perry, one of the most accomplished mechanical and optical technicians in making diffraction gratings, constructs parts of the ruling engine, including the diamond used to cut the 75,000 grooves on the aluminum-coated glass. Diffraction grating is the basis of spectroscopy, separating light into various colors for analysis.