John Herschel's drawing of an actinometer, c 1830s.
© Science Museum / Science & Society Picture Library
Experiment 969 from John Herschel's own notebook. An actinometer is an instrument for measuring the intensity of radiation, especially ultraviolet radiation. Herschel (1792-1871) was an English astronomer, mathematician and chemist who discovered 525 nebulae and clusters. He pioneered celestial photography and invented the cyanotype or blueprint proces.