  We present a Michelson-Morley type experiment for testing the isotropy of the
speed of light in vacuum and matter. The experiment compares the resonance
frequency of an actively rotated monolithic optical cryogenic sapphire
resonator against the resonance frequency of a stationary evacuated optical
cavity made of ultra-low-expansion glass. The results yield an upper limit for
the anisotropy of the speed of light in matter (sapphire) of Delta(c)/c < 1 x
10^-16, limited by the frequency stability of the sapphire resonator.
