  We present an improved laboratory test of Lorentz invariance in
electrodynamics by testing the isotropy of the speed of light. Our measurement
compares the resonance frequencies of two orthogonal optical resonators that
are implemented in a single block of fused silica and are rotated continuously
on a precision air bearing turntable. An analysis of data recorded over the
course of one year sets a limit on an anisotropy of the speed of light of
\Delta c/c ~ 1 x 10^{-17}. This constitutes the most accurate laboratory test
of the isotropy of $c$ to date and allows to constrain parameters of a Lorentz
violating extension of the standard model of particle physics down to a level
of 10^{-17}.
