Afferent pupillary defect of the right eye, demonstrated by the swinging flashlight test

An afferent pupillary defect occurs in optic neuritis if the other eye is uninvolved and otherwise healthy. This is demonstrated by shining a light alternately in one eye and then the other and finding that the direct response to light is more sluggish in the affected eye. Alternate swinging of the light between the two eyes therefore produces dilation each time the light is directed to the affected eye.
(Image A) With both eyes exposed to the same amount of light, the pupils are equal because the efferent pathways are intact.
(Image B) When the light is directed to the unaffected (left) eye, both eyes constrict normally because light is detected normally by the left eye.
(Image C) When the light is directed to the right eye, it perceives little or no light because the afferent pathway on that side is damaged, and both pupils dilate.