speeds and orientations, de Sitter concluded that the effect should have been seen if the model was correct, and its absence meant that the emission theory was almost certainly wrong. Notes Modern experiments of the de Sitter type refute the idea that light might travel at a speed that was partially dependent on the velocity of the emitter (c'=c + kv), where the emitter's velocity v can be positive or negative, and k is a factor between 0 and 1, denoting the extent to which the speed of light depends on the source velocity. De Sitter established an upper limit of k < 0.002, but extinction effects make that result suspect. De Sitter's experiment was criticized because of extinction effects by J. G. Fox. That is, during their flight to Earth, the light rays would have been absorbed and re-emitted by interstellar matter nearly at rest relative to Earth, so that the speed of light should become constant with respect to Earth, regardless of the motion of the original source(s). In 1977, Kenneth Brecher published the results of a similar double-survey, and reached a similar conclusion - that any apparent irregularities in double-star orbits were too small to support the emission theory. Contrary to de Sitter, he observed the x-ray spectrum, thereby eliminating possible influences of the extinction effect. He established an upper limit of . There are also terrestrial experiments that speak against such theories, see experiments testing emission theories. References Physics experiments 1913 in science