Robert Marcus Gunn was a Scottish physician from the 1800’s. He studied under other well known physicians such at Douglas Argyll Robertson. Dr. Gunn is known for the Marcus Gunn pupil (AKA relative afferent pupillary defect or RAPD).
The Marcus Gunn pupil can be detected by swinging a flashlight between both eyes. Of course, normally, if you flash light in one eye, both pupils will constrict. However, if one eye has a problem with detecting light (usually from a problem with the retina or optic nerve), there will be less constriction when light is shown on that eye. Sometimes there may be no constriction or even paradoxical dilation in one or both eyes!
This can be seen in disease of the retina or optic nerve such as in retinal detachment, retinal ischemia or optic neuritis, among other causes.