IMAGE PROBLEMS

For all of these improvements, there was still no way to capture the camera obscura's image other than by manually tracing it. There it was, tantalizingly projected onto a wall or a pane of frosted glass. You could look at it; you could reach out and touch it. But capturing the actual image was as impossible as capturing one's own shadow. It would remain so for another 75 years...until the invention of film.

SPIRITS IN THE MATERIAL WORLD

In 1674 an alchemist named Christoph Adolph Balduin performed a chemistry experiment that he hoped would help him isolate the mysterious natural force he called the "universal spirit." He dissolved some chalk (calcium carbonate) in nitric acid to create a sludgy substance that would easily absorb moisture from the atmosphere. Balduin believed that if he could distill the moisture from the sludge, he would capture the universal spirit in pure form.

Balduin did not know much about chemistry, and not many 17th-century alchemists did. When he distilled the sludge, all he got was water. However, he noticed that when he heated the dried-out crud that was left over, it glowed in the dark. He named this mysterious substance phosphorus, Greek for "bringer of light," (today it is called calcium nitrate).

What did this have to do with photography? Nothing... until a German anatomy professor named Johann Heinrich Schulze tried to repeat Balduin's experiment in 1727. By chance he used nitric acid that contained traces of silver. He left the chalk-acid mixture out in the sun; by the time he came back to it, it had turned a deep purple.

MESSAGE IN A BOTTLE

Schulze was not the first person to observe that substances containing silver salts turn dark when exposed to the sun.
However, it had always been assumed the reaction was caused by the heat. Schulze suspected that light was to blame and came up with an experiment to test his theory: He cut a stencil of some words on a piece of paper. He put the stencil on the side of a glass bottle and covered the rest of the glass with dark material. He filled the bottle with the chalk dissolved in nitric acid and left it out in the sun, to see if the sunlight would "write" the stenciled words onto the material.

 "It was not long," he wrote later, "before the sun's rays, where they hit the glass through the cut-out parts of the paper, wrote each word on the chalk precipitate so exactly and distinctly that many who were curious about the experiment took occasion to attribute the thing to some sort of trick." In a nod to Balduin, Schulze called the material scotophorus, or "bringer of darkness."

Schulze did not understand why the substance turned dark, but today we do: When light strikes photosensitive silver crystals, some of the atoms of silver separate out from the compound. Exactly how many atoms separate depends on how much light strikes the material. With enough light, however, the silver will become visible to the naked eye, and the material becomes dark. This is the chemical principle upon which all film photography would be based.

Schulze could not figure out how to control the reaction—the silver salts darkened every time they were exposed to light, obliterating whatever writing or image had been created. As far as he could tell, the material had no use, but it was still interesting, and as word of his discovery spread, scientists all over Europe repeated the experiment.

CAMERA OBSCURA

 The ancient principle of the camera is child's play. Hard to believe? Here is a simple experiment you can try at home:

Cover the windows of a room with black construction paper or aluminum foil until absolutely no light is let in. Turn out the lights. Then poke a tiny hole in the paper or foil, so that a single pinprick of light enters the room and strikes the wall opposite the windows. What do you see?
If you do it just right, when the light enters the "dark room" (camera obscura in Latin) and hits the wall, it will form a faint upside-down image of the view outside the window. This simple phenomenon is the basis upon which the science of photography is built.

One of the first people to make note of such an image was a Chinese scholar Mo Ti, who lived in the 5th century B.C. In the 10th century A.D., Arab physicist Alhazen discovered that the smaller he made the hole, the sharper the image came into focus. If the hole was tiny enough, the image became very clear.

THROUGH THE LOOKING GLASS

 Reproducing the image created by a camera obscura was easy: you simply held a piece of paper up against the wall, so that the image landed on the paper, then traced it. The camera obscura became a useful scientific tool. Scientists built special "dark rooms" for the sole purpose of studying the sky, eclipses, changes in the seasons, and other natural events.

The tracings made with the aid of the camera obscuras were so detailed and accurate that by the 1500s, people were using them to paint portraits, landscapes, and other scenes.
In 1568 a professor at the University of Padua named Daniello Barbara discovered that replacing the primitive pinhole with a glass lens brought the camera obscura image into a brighter and sharper focus.
In the 17th century, scientists and artists developed portable camera obscuras that allowed them to study objects in the field. Early versions were essentially lightproof tents with lenses sewn into the walls. Later versions were two-foot-long wooden boxes that projected an image onto a piece of frosted glass built into the lid. The user could then trace the image by placing a piece of paper over the glass.

PUTTING THINGS IN PERSPECTIVE

 The images created by these early single-lens camera obscuras were circular in shape, with distortion along the edges. In the 1700s, a complex multilens system was introduced that corrected the distortion, and the camera obscura became as common a part of the painter's art as brushes and paint.
Artists were not the only ones putting the camera to use—explorers took them on expeditions all over the world so that they could record the wonders they encountered. In the process, the boxes literally changed the way people saw the world.