Extremely sensitive readout of a stable "etalon of length" is achieved with laser frequency-locking techniques. Rotation of the entire electro-optical system maps any cosmic directional anisotropy of space into a corresponding frequency variation. We found a fractional length change $\frac{\Delta l}{l}=(1.5\mathrm{}\pm 2.5)\times {10}^{-15\mathrm{}}$, with the expected $P_2\left(cos\theta \right)$ signature. This null result represents a 4000-fold improvement on the best previous measurement of Jaseja et al.

• Received 20 November 1978

DOI:https://doi.org/10.1103/PhysRevLett.42.549