NEWTONS ZERO-ORDER RAINBOW - UNOBSERVABLE OR NONEXISTENT

Newton asserted without proof that a zero-order rainbow exists, one associated with two refractions and no reflection. Subsequent authors also postulated such a rainbow but argued that it is not seen because its angular position (26 deg) is too close to the Sun. But it is not seen because it does not exist. Newton's erroneous 26-deg rainbow appears to have resulted from his failure to account for the dependence on angle of incidence of the solid angle of twice-refracted rays corresponding to a given bundle of incident rays.