Slit-lamp photograph of the cornea of an eye affected with a dendritic (branching) ulcer. A yellow fluorescein dye has been used to show the extent of the ulcer across the cornea (the transparent circular covering of the front of the eyeball), with the main body of the ulcer (running from top to bottom) stained with rose bengal. Dendritic ulcers are caused by the herpes simplex virus, and tend to recur because of the ability of the virus to lie dormant in tissues; years may elapse between successive attacks. Magnification: X 1.6.