A high voltage was passed through a stick of calcium carbonate (lime) which was placed in front of a reflector dish. The lime glowed with an intense light which was then focused down onto the actor on stage.
Limelight is an intense white light produced by heating LIME inan oxyhydrogen flame. It was used long ago in theatres for lighting purposes. From this early usage the secondary meaning of publicity, and full glare of media attention was derived.
A theatrical spotlight and also a popular term for incandescent calcium light invented by Thomas Drummond in 1816. Drummond's light consisted of a block of calcium heated to incandescence in jets of burning oxygen and hydrogen.