IN an earlier paper1, we have described a new type of interferometer which has been used to measure the angular diameter of radio stars2. In this instrument the signals from two aerials Ax and A2 (Fig. la) are detected independently and the correlation between the low-frequency outputs of the detectors is recorded. The relative phases of the two radio signals are therefore lost, and only the correlation in their intensity fluctuations is measured ; so that the principle differs radically from that of the familiar Michelson interferometer where the signals are combined before detection and where their relative phase must be preserved.

This new system was developed for use with very long base-lines, and experimentally it has proved to be largely free of the effects of ionospheric scintillation2. These advantages led us to suggest1 that the principle might be applied to the measurement of the angular diameter of visual stars. Thus one could replace the two aerials by two mirrors M19 M2 (Fig. 16) and the radio-frequency detectors by photoelectric cells Gl9 C2, and measure, as a function of the separation of the mirrors, the correlation between the fluctuations in the currents from the cells when illuminated by a star.

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