Toothed wheels and rotating mirrors: Parisian astronomy and mid-nineteenth century experimental measurements of the speed of light
The speed of light was first shown to be finite by Rømer in 1676 at the Paris Observatory. A century and a half later the Observatory was again a centre of research on the speed of light—but this time in the laboratory or across Paris. Arago and Le Verrier, successive Observatory directors, encouraged Foucault, Fizeau and Cornu to rotate mirrors or spin toothed wheels. In an experiment devised by Arago, light was found to travel faster in air than water, completely demolishing the corpuscular theory, which had already been undermined by a number of physical experiments and astronomical observations. Subsequent absolute measurements supported the lower value for the speed of light and smaller distance to the Sun suggested by Le Verrier's analysis of solar-system motions.