The Camera Obscura

The principal of the camera obscura had been known since ancient times. It was first detailed in writings by Leonardo da Vinci.  Meaning literally "darkened room," it was originally a room completely sealed from light except for a very small hole in one wall. An image of the outside world - houses, trees, and even people - could be projected, upside down and reversed right-to-left, onto a wall or white screen placed opposite the opening. Later the camera obscura was reduced in size until it became a small portable box. It was equipped with a lens and a mirror at a 45-degree angle, which reflected the image upward and focused it on a viewing screen. This was a great aid to artists in making sketches on location, but there was not yet a way to capture directly and permanently the camera obscura's images.

In 1038 AD, an Arab scholar named Alhazan described a working model of the camera obscura. Although Alhazan did not actually construct the device, his work would influence Roger Bacon who, in 1267 AD, created convincing optical illusions by using mirrors and the basic principles of the camera obscura. Later, he used a camera obscura to project an image of the sun directly upon an opposite wall. Throughout the middle ages, Bacon's ideas were adapted for astronomical observations of the  sun. The camera obscura became a popular tool for safely viewing eclipses.

Aidan Campbell, a pop artist, as put forth the proposition that all great artists of the 17th century used a Camera Obscura, in secret, to trace reality onto the canvas and this is why their vivid representations are so perfect. He also goes on to state that with the invention of photography, artists became more amenable to the distortions because it had become pointless by then to trace lens-produced portraits in secret.

More Info
http://www.acmi.net.au/AIC/CAMERA_OBSCURA.html
http://brightbytes.com/cosite/what.html