John Tyndall, Irish physicist, c 1860.

IRELAND - JULY 18: John Tyndall (1820-1893) was a surveyor and civil engineer in Ireland. After a brief period as a schoolmaster he left to study sciences in Berlin, Germany. In the 1850s he began some pioneering studies on glacier formation and, in 1856, became the first man to climb the Weisshorn. In 1859 he began research on heat radiation, followed by the acoustic properties of the atmosphere and the blue colour of the sky. His research resulted in the discovery of the 'Tyndall Effect', whereby a beam of light is made visible by the scattering of light by fine particles in the air and in liquids. A talented lecturer and prolific writer on scientific subjects, he died from accidental poisoning with chloral in 1893. (Photo by SSPL/Getty Images)