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A spectrograph has been constructed which consists of a wide slit, attached to the outside of the upper end of the tube of the 4o-inch telescope, and an f : i Schmidt camera of 94 mm aperture with two 600 quartz prisms. Very faint nebulae can be ob- served with this instrument, provided their diameters exceed ii'
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Otto Struve (1937) studied this question.
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