[Muscae volitantes--from early observations to Purkinje's explanation]

The subjective visual phenomena known as "muscae volitantes" ("floating flies") were already known to the Greeks. Galen (2nd c. A.D.) explained them within his theory of vision as circumscript condensations of the aqueous. This opinion prevailed in Arab medicine and even in the science of the Renaissance and later. When, in the 17th c., Galen's theory, according to which the crystalline lens was the organ of visual perception, was replaced by Kepler's insight that the image of the outer world is formed on the retina, scholars such as Dechales and Morgagni came near the truth. In 1823, Purkinĕ identified the muscae as opacities floating in the vitreous.