Blue light.

About Blue light

Blue light is an archaic signal, the progenitor of modern pyrotechnic flares. Blue light consists of a loose, chemical composition burned in an open, hand-held hemispherical wooden cup, and so is more akin to the flashpan signals of the Admiral Nelson era than the modern, encased signal flares, which are often launched by mortar or rifle and suspended by parachute. Widely used during the eighteenth and nineteenth centuries for signaling by the world's military forces, and for general illumination in the civilian sector, blue light was remarkable for its use of poisonous arsenic compounds, which contributed to its replacement by safer flares in the early twentieth century.