Limelight projector, early 20th century.
© Science Museum / Science & Society Picture Library
In 1825, it was discovered that a piece of limestone became incandescent and glowed brilliantly if heated by an oxy-hydrogen flame. In theatres from the 1850s, lenses were placed in front of the lime, producing a spotlight effect for use on stage. In the later example shown here, the oxygen and hydrogen were led by tap-controlled pipes to a backward sloping nozzle from which the flame impinged upon a cylinder of lime mounted on a central pin. The cylinder could be turned by hand to present a fresh surface to the flame.