| Regardless of the source or kind of
light, the ability of light to form an image is dependent upon just one thing--a
small hole or a lens to bring the rays to a focus. The principle of image
formation was discovered in early Greek Times. Later it was realized that the
eye itself has a lens and that an image is, in fact, formed on the retina, the
rear inside surface of the eyeball. Some man inside a cave or a darkened room
noticed that light entering through a small hole in the wall formed an image on
the far wall. It was observed that if you enlarge the hole, the image blurred
and disappeared. On the other hand, the smaller the hole, the sharper the
image-down to a certain diameter after which making the hole smaller makes the
image worse again. The function of a pinhole is to screen out all light rays except those coming in a perfectly straight line from the object, thus improving the sharpness of the image. After passing through a point the rays again spread out a little bit, and the size of the image formed will depend upon how far back the wall, screen, or film is from the pinhole. The full exploitation of this phenomenon had to wait for the development of the lens, which gathered in a much larger number of light rays and still brought them to form an image, just as the pinhole did, but with a difference: in a pinhole camera, the light rays form an image that is equally sharp regardless of the distance to the film. When a lens is used, there may be a choice of planes of focus determined by adjusting the lens-to-film distance. Each plane is located some distance behind and relatively parallel to the lens. Actual blown glass was developed early in Egyptian times, and the first lens may have been the bottom of a wine bottle. However, the first deliberate grinding of lenses did not take place until the thirteenth century, and the art did not become established until the sixteenth century. A book on the grinding and polishing of lenses was issued by B. Battista della Porta in 1589, but it was not until 1611 that Kepler compared a lens of glass to the lens in the eye and showed that rays from each point of an object were brought to a focus at each corresponding point of an image on the retina. Then, in 1619, Scheiner demonstrated the actual formation of an inverted image on the retina. |