What is a system of close equidistant and parallel lines lines or bars, especially lines ruled on a polished surface, used for producing spectra by diffraction; -- called also diffraction grating called?

grating — (Optics) A system of close equidistant and parallel lines lines or bars, especially lines ruled on a polished surface, used for producing spectra by diffraction; -- called also diffraction grating. (Webster's 1913.)