Limelight, developed in the 1820s by Goldsworthy Gurney and popularised in theatres from the 1840s onward, was produced by directing an oxy-hydrogen flame onto a piece of calcium oxide (lime), which glows with an intensely bright, white light. It was the first directional, powerful artificial light source available to theatre — and it transformed performance. For the first time, a single actor or dancer could be isolated from the rest of the stage by a concentrated beam of light, creating dramatic focus impossible with candles or gas. The term 'in the limelight' entered the language as a metaphor for being the centre of public attention. It was gradually replaced by electric arc lights in the 1880s and early 1900s. Its legacy — the principle of a directed spotlight isolating a performer — remains the fundamental grammar of theatrical lighting.