Spectroscopic gratings are generally small, measuring only a few centimetres per side. Until recently, they were ruled using a ruling engine — the most precise mechanical device prior to about 1965. A ruling engine moved a diamond across a piece of optically polished glass, cutting or scoring very fine lines into the surface, one line at a time. A typical grating would have 5,000 to 12,000 lines per centimetre (12,500 to 30,000 lines per inch). Some gratings had almost 200,000 lines per centimetre (500,000 lines per inch).

Blythswood dividing engine, ca 1912

The diamond is shaped to rule the grooves one at a time. (Babcock, Vistas in Astronomy, 1986)

During the scoring process, the automatic ruling engine had to operate flawlessly in a temperature-controlled environment for hours, or even days. The slightest vibration of the machine, a glitch in the gearing, the smallest change in temperature or a chip off the ruling diamond could make the entire ruling useless. The most critical element of the whole machine was the lead screw, which moved the diamond from one line to the next, and the slightest error in the precision of the screw would show up on the grating.

Light reflecting from the grooves of a grating

Spectrographs with gratings or prisms are used by scientists — astronomers, chemists and physicists — to study the chemical composition and conditions of gases and solid materials. Each gas or substance has a unique “bar code”, or spectrum, which tells us about the conditions in which it exists. For astronomers who can never visit their “labs” — stars, nebulae or galaxies — spectra are their only way of studying such objects.