Acquired vision

Acquired vision is the phenomenon of a blind person gaining the ability to see, usually as a result of medical treatment.

Medical
The first known case of acquired vision is in 1728, of a blind 13 year old boy by William Cheselden. In 1960, Maurice von Senden restored vision to 65 patients with congenital cataracts. Corneal grafts are also becoming more common. More recently, another condition called aniridia has been treated using the amniotic sac that surrounds a fetus. In 2003, three people were successfully implanted with a permanent "retinal prosthesis" by researchers at the University of Southern California. Each patient wore spectacles with miniature video cameras that transmitted signals to a 4-mm-by-5-mm retinal implant via a wireless receiver embedded behind the ear.

As a thought experiment
John Locke, an 18th century philosopher, speculated that if a blind person were to develop vision, he would not at first connect his idea of a shape with the sight of a shape. That is, if he was asked which was the cube and which was the sphere, he would not be able to do so, or even guess. This thought experiment (it was a thought experiment at the time) outlines the debate between rationalism and empiricism; whether our knowledge of the world comes from reason or experience. In 1749, Denis Diderot wrote Letter on the blind for the benefit of those who see as a criticism of our knowledge of ultimate reality.

Michael G. May
Michael G. May had a stem-cell transplant in his right eye in 2001 when he was 43, after 40 years of blindness. He reportedly has adapted well to his recovered vision.
 * May still has no intuitive grasp of depth perception. As people walk away from him, he perceives them as literally shrinking in size
 * problems distinguishing male from female faces, and recognizing emotional expressions on unfamiliar faces.

Virgil
In his book, An Anthropologist On Mars (1995), neurologist Oliver Sacks recounts the story of Virgil, a man who saw very little until having cataract surgery at age 50. Virgil's subsequent behavior was that of a "mentally blind" person —someone who sees but can't decipher what's out there; he would act as if he were still blind. Often confused, Virgil rapidly sank into depression. About 4 months after his surgery, he died of pneumonia. 

Sidney Bradford
In 1974, Richard Gregory described a patient, Sidney Bradford, a 52 year-old who gained vision from corneal grafts to both eyes. Before surgery Bradford was a skilled machinist, but upon gaining vision, he became confused and unable to work. He committed suicide 2 years after his operation.

Misc

 * There is a biblical description of this phenomenon. In Mark 8:22–26, a blind man reports after an initial healing touch by Jesus that he sees people, but they look like "walking trees." After a second healing touch, the man sees everything "clearly."