Duke-Elder,1 in discussing the clinical picture of optic neuritis, mentions "a peculiar pupillary reaction common to all forms of conduction interference, wherein, although both the direct and consensual reactions are present the contraction is not maintained under bright illumination so that the pupil slowly dilates again while the light is still kept upon the eye." This seemingly paradoxical light reaction of the pupil was observed in retrobulbar neuritis by Marcus Gunn (1904), who attributed it to the consensual reaction of the pupil to darkness. In this paper is presented a simple and sensitive method for demonstrating the Marcus Gunn sign and, also, a hypothesis to show that there is nothing peculiar or paradoxical about this pupillary reaction, which should be expected whenever the light sense is reduced by disease of the retina or optic nerve. Hypothesis Assume that the pupillomotor impulses produced by visible light (and hence the stimuli