Photography is the technique of recording, by chemical or mechanical means, a permanent image on a layer of material sensitive to light exposure. The understanding that prevails today assumes the use of a camera or camera obscura as the image forming device, and of photographic film as the recording medium, but it doesn't have to be the case. For instance, the photocopy or xerography machine is forming permanent images from a brightly lit original, but is using the transfer of static electrical charges rather than photographic film, hence the term electrophotography. Whereas the rayographs published by Man Ray in 1922 are images produced by the shadows of objects cast on the photographic paper, without the use of a camera.
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About the same time in England, Fox Talbot discovered the process of developing and fixing the exposed image as a negative and making a positive print. Scott Archer (1813-57) in 1851 invented the collodion 'wet plate', and dry plates coated with sensitized emulsion were produced commercially in the mid-1870s, followed by celluloid-based film from 1889.
In a camera, light reaching the photo-sensitive emulsion containing silver halide crystals forms a latent (invisible) image, which can be made visible by chemical development, reducing the exposed crystals to black metallic silver. The remaining unaffected halide is then removed by fixing to leave a permanent negative record of the exposure.
By exposing another photo-sensitive material to light passing through this negative, a print can be made which, after developing and fixing, yields a positive image representing the original scene. In a reversal system, the film exposed in the camera is processed to produce a positive rather than a negative image by removing the initially exposed halide.
