It was at Clark that Michelson conducted fundamental research leading to the determination of an international standard of measurement, the meter, in terms of a natural constant, the wave length of cadmium light. His numerous contributions to the physics of light included the invention of optical precision instruments, many of which he used in experiments that marked the beginning of modern physics. Albert Einstein, in 1931, publicly paid tribute to Michelson's extensive contributions to science: "My honored Dr. Michelson, it was you who led the physicists into new paths, and through your marvelous experimental work paved the way for the development of the theory of relativity."