William Herschel's heliostat, 1795-1800.
© Science Museum / Science & Society Picture Library
Sir William Herschel (1738-1822), the famous astronomer, used this heliostat for his experiments on thermal radiation. He used the mirror to direct sunlight through the glas prism also shown. This produced a spectrum whose temperature was les towards the blue than the red and greatest beyond the visible red light. Known today as the infrared this was the first time that radiation beyond visible light had been detected by experiment.