The theory of photography originated from the discovery of the phenomenon of the camera obscura: light entering a dark room through a small hole projects an identical image upside down on the internal wall of the scene external. The first records of scientists recognizing this concept were in the writings of the Greek philosopher Aristotle (384 – 322 BC). Early models of the Camera Obscura were large chambers that the artist could enter. Initially, this invention was recognized as an aid for artists who could trace images to create a more realistic impression of the scene. The difficulty with the camera was that it was not easily transportable and was therefore useless to an artist. This problem was solved when progress was made in the 17th century, when inventors developed a portable version of the optical device. Furthermore, those who used the tool discovered that the image produced was inaccurate as it defied the rules of perspective because it was made up of a single lens. The inventors discovered a way to correct this problem, as explained in The History of Photography: “By combining two lenses ground on segments of arcs of different radii a reasonable flatness of field could be achieved. And by using lenses of different focal lengths, the angle of view could be narrow for portraits and wide for landscape work” (Newhall 9). As the use of this tool became widely practiced among artists, experimenters began to realize the possibilities of the invention. The idea that images formed by the Camera Obscura could be saved as permanent prints came to light in 1790, when Thomas Wedgwood began experimenting with light-sensitive silver salts. The discovery of the effects of light on certain chemicals was made by… half the paper… five prints by Brady and his team amounted to over seven thousand. In contrast to Fenton's romantic style, Brady's photos told a much more graphic story, influencing Americans everywhere. Other important photographers whose prints moved the public were Jacob August Riis (1849 – 1914) and Lewis W. Hine (1874 – 1940). At the age of twenty-one, Danish photographer Riis emigrated to the United States, where he encountered the poor side of New York City. Four years after his arrival, Riis was assigned the position of police reporter. He desired social reform and published a book describing slum conditions, illustrated with drawings based on his original photographs. Lewis Hine photographed factories and children working in coal mines in an attempt to raise awareness among the upper classes, leading to the downfall of child labor in America.