Interference Methods in Astronomy 145
distant that this limit of resolution is passed, the telescope can give no information concerning them.
2. But an observation of the visibility curves of the interference fringes due to such sources, when made by the method of the double slit or its equivalent, and properly interpreted, gives information concerning the size, shape, and distribution of the components of the system. Even in the case of a fixed star, which may subtend an angle of less than one-hundredth of a second, it may not be an entirely hope- less task to attempt to measure its diameter by this means.