Special relativity's stipulated independence of light velocity from source velocity has seldom been tested in the macroscopic scale, due to the difficulties encountered in the devising of a method. Since QSOs are the most extreme entity known in both velocity and scale, they constitute the ideal tool for such a test. The experiment can be conducted by taking a series of plates of QSOs, spread over the course of a year, and then looking for an elliptical displacement of the QSO relative to the stellar foreground. If a displacement is found, and it is oriented in the expected sense, it may be concluded that c has varied by a proportional amount. Direct images of four QSOs have confirmed the constancy of c to within 0.4 percent, for relativistic source velocities over a scale approaching the size of the universe.