Calcium Atomic Beam Source and Interference beyond Two-Meter Retardation*

Fabry-Perot interference patterns of the resonance line of calcium, λ4226 A, have now been obtained at retardations of 153 and 204 cm by employing a highly collimated calcium atomic beam source in emission. The corresponding order numbers are 3.62 and 4.82 million. The appearance of these fringes suggests that the half-width of the line is less than 3×10−3 cm−1 which is one-fourth that of the proposed primary standard of length and one-tenth that of the present primary standard of wavelength. The use of an atomic beam source for the production of the length standard is discussed.