William Herschel's glas prism on stand, 1780-1800.
© Science Museum / Science & Society Picture Library
Sir William Herschel (1738-1822), the famous astronomer, used this glas prism with bras base for his experiments on thermal radiation. Used in conjunction with a heliostat, Herschel directed sunlight through the prism. 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.