Interference at Great Retardations Obtained with a Calcium Atomic Beam Source*

By employing a calcium atomic beam source in emission and by using a beam of high collimation very clear Fabry-Perot patterns of the resonance line of calcium, λ4226 A, were obtained with retardations of 60, 78, 94, 108, 120, and 130 cm. The corresponding order numbers are about 1.42, 1.85, 2.22, 2.56, 2.84, and 3.08 million and greatly surpass those obtainable with the best conventional cooled source available at present. The good definition of the fringes obtained with the retardation of 130 cm suggests that the limit of interference is considerably higher.