In view of the far-reaching consequences of Einstein’s principle of relativity it is quite remarkable how few direct experimental tests of this principle have actually been performed. The classical experiments by Michelson (1881), and by Michelson & Morley (1887), on which the theory of relativity was based, date back to the end of the last century. Michelson & Morley utilized an interferometer arrangement which should allow the detection of a possible influence of the absolute velocity of the laboratory on the velocity of propagation of light. In accordance with the principle of relativity, no such influence was revealed by these experiments. However, this result was contested by Miller (1933), and even the later more accurate experiments of this type, performed by Joos as late as in 1930, provided only an upper limit for the ether drift of 1.5 km/s. Although this value is a good deal smaller than the 30 km/s of the earth’s motion around the sun, it is perhaps fair to say that if the principle of relativity had to be based on these experiments only, its foundation would be somewhat shaky. It is true that by now we have good experimental verifications of a large number of special relativity effects which ultimately are based on the principle of relativity. Nevertheless, it would be desirable to have other direct tests of this principle with a higher accuracy than the experiments of the Michelson-Morley type.