retinal capillaries. The patient is alternatingly shown blue light and a computer generated picture of moving dots; by adjusting the speed and density of these dots, the patient tries to match the computer generated picture to the perceived entoptic dots. Difference from other entoptic phenomena Scheerer's phenomenon can be easily distinguished from floaters (muscae volitantes). Scheerer's phenomenon consists of corpuscles of identical diameter and visual sharpness, of a simple dot or worm-like shape, brighter than the background. If the eye stops moving, the dots keep darting around. If the eye moves, the dots follow instantaneously, because they are contained in the retina. In contrast, floaters are specks or threads of variable diameter and variable visual sharpness, some of complex shape, darker than the background. If the eye stops moving, the floaters settle down. If the eye moves, the floaters follow sluggishly, because they are contained in the vitreous humor, which is subject to inertia. Scheerer's phenomenon can be distinguished from visual snow because it appears only when looking into bright light, whereas visual snow is constantly present in all light conditions including the dark. See also Entoptic phenomenon Floater References External links Video describing history and science of blue field entoptic phenomenon and viewing techniques Optical phenomena Ophthalmology Vision