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A CAUTIONARY NOTE ON THE USE OF PICKERING-RACINE PRISMS.

The value of Δm, the magnitude difference between primary and secondary images in photographic plates exposed with Pickering-Racine prisms, is found to depend, at least in some observations, on the magnitude of the primary image if the plates are measured with an iris-diaphragm astrophotometer. The effect may invalidate the extrapolation of photometric sequences when such measurements are used.