A healthy fluorescin-treated eye under ultraviolet light. Fluorescin is a harmless orange dye (seen here glowing green in u.v.) used in ophthalmology as an aid to the diagnosis of certain types of disorder. Fluorescin drops are applied to the eye and abrasions to the conjunctiva (the delicate mucous membrane which covers the front of the eye and lines the inside of the eyelids) fluoresce under ultraviolet light. Abrasions and ulceration of the cornea (the transparent wall of the eye beneath the conjunctiva) show up similarly. In this healthy subject only the margins of the eye are fluorescing where the dye collects and drains away with the tears.