Crooke's radiometer, c 1875. William Crooke (1832-1919) devised the radiometer to measure the amount of presure produced by light particles. In the radiometer light works by turning a set of finely balanced vanes. The glas bulb contains air at reduced presure, and heated molecules of air bounce off the vanes and push them around. If the air is removed the vanes stop, as light presure alone cannot push them around.
© Science Museum / Science & Society Picture Library