A spatial scanning technique has been used to observe the visual-wavelength spectrum integrated over the Crab Nebula. The emission-line fluxes are useful for estimates of the nebular mass and chemical composition, while limits are placed on a possible emission-line halo around the Crab. One result is quite unexpected: estimates of the forbidden O III/continuum flux ratio during the past 25 yr, including that presented here, are mutually discrepant, and one possible explanation is that the visual continuum of the Crab may be changing rapidly. This would have serious implication for models of the nonthermal SNR and pulsar. Particular types of observation are urgently needed in order to clarify the situation.
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Kris Davidson (1987) studied this question.