Arago's prism experiment — The refractive index of a glass prism depends on the incident velocity of light outside the prism and its velocity inside the prism after refraction. On Fresnel's fixed ether hypothesis, the incident light waves are situated in the stationary ether outside the prism and move with velocity 0 with respect to the ether. If the prism moves with a velocity u with respect to this fixed ether, then the incident velocity bf light with respect to the prism should be c+u. Thus the refractive index of the glass prism should depend on a, the absolute velocity of the prism, i.e., its velocity with respectto the fixed ether. Arago performed the experiment in 1819, but failed to detect the expected change.