Camera Obscura - How it Worked
he principle of what came to be called, around the year 1600, the camera obscura, was known in the fifth century before the Christian Era, when a Chinese philosopher discovered that light passing through a pinhole produces an inverted image on the opposite wall of a darkened room. A century later the Greek philosopher Aristotle made similar observations. For the next eighteen centuries, applications of the principle were limited to intermittent scientific experiments until, in about 1550, a biconvex lens was added, along with a diaphragm to control the size of the aperture and improve sharpness of focus and depth of field.1
In 1676 a German mathematician added a mirror to re-invert the image onto a glass plate, upon which anyone could copy the image on translucent paper. In 1685 a German monk introduced interchangeable lenses of various focal lengths, to accommodate a broader range of subjects, from wide-angle landscapes to closeup portraits. Meanwhile, camera obscuras had become small enough to be easily portable, like the one pictured above, and were popular throughout Europe and America among artists as well as amateurs.
There is no accounting for Lewis's use of crimee for camera; its Clarkian spelling is not even close to a phonetic rendering of the Italian word. Thomas Jefferson purchased a camera obscura of his own in 1794, and Lewis may have seen his, or even used it.2 The machine was listed in Owen's Dictionary, the four-volume reference work that was part of the expedition's baggage, although the example described therein is not identical with either of the styles pictured above.3 Perhaps crimee was but the slip of a hasty pen.A camera obscura was among the equipment carried by the Freeman-Custis Expedition to the Red River of the South in the spring of 1806, but no images made with it are known to exist. Later, several large-scale camera obscuras were built for education and entertainment.4
During the 1830s a French painter and physicist named Louis-Jacques-Mandé Daguerre invented the first practical process for capturing an image on a copper — instead of glass — plate coated with silver iodide, fuming it with mercury vapor, and then bathing it in a common salt solution to make the image permanent. The result was known as a daguerreotype. In 1839 the British scientist Sir John Herschel called it by a new name, photograph — from the Greek words photos ("light") and graphein ("to draw").
The invention of photography confirmed what users of the camera obscura as a drawing tool had long since learned, that whereas the human eye sees what its owner chooses to notice, the lens presents all visible details of a scene indiscriminately.5 The lens makes the choice, stops the clock, and engages the viewer detail by detail. That is why the camera obscura was used by military spies. And perhaps that is why Meriwether Lewis wished he had brought one along, "by the assistance of which even I could have hoped to have done better."
1. See "FotoHistory: Camera Obscura" at http://www.spress.de/fofo/history/optics.htm/ (accessed 12/00).
2. Jefferson wrote to David Rittenhouse in September of 1793, asking in behalf of two young ladies who were his houseguests, for the loan of Rittenhouse's camera obscura, "that they may take a few lessons in drawing from nature." The Papers of Thomas Jefferson (29 vols., Princeton, New Jersey: Princeton University Press, 1997), vol. 27, p. 47.
3. "For the construction of a portable camera obscura, the box, or chest, must be in breadth and length proportionable to the different magnitude of the diameter of the lens. In one of the sides fix a lens, and white paper on an opposite glass, at a proper distance; and having made a little hole near the glass, you may, thro' that, see the images of the objects, in a beautiful manner, on the paper." Published in London in 1753 (2nd ed., 1764), the dictionary was known as Owen's Dictionary, after the publisher. The full title is A New and Complete Dictionary of Arts and Science; comprehending all the branches of useful knowledge, with accurate descriptions as well of the various machines, instruments, tools, figures, and schemes necessary for illustrating them, as of the classes, kinds, preparations, and uses of natural productions, whether animals, vegetables, minerals, fossils, or fluids . . . by a Society of Gentlemen. Vol. 1.
4. See "Pinhole Photography" at http://home.sol.no/Etion/pinhole.htm/ (accessed 12/00).