Just for a change, I thought we'd take a look at an old piece of technology. It is something that was cutting edge this time last century, but now seems, finally, to be on its way out. What is widely acknow-ledged to be the world's first photograph was taken in 1827, but it wasn't until George Eastman had the idea of making roll film in 1889 that photography became something accessible, if only to a select group of people. The prototype of today's film was made of large, delicate plates of glass. The first colour film was invented by the Lumiere brothers in 1907, and the first widely sold colour film was marketed by Kodak in 1935. NO MATTER WHAT type of film you use, colour or black and white, negative or slide, it all works in basically the same way. A piece of film has two parts, emulsion and celluloid. Celluloid is a transparent plastic that constitutes the base of the film. It is what Eastman used to make that first roll film in 1889. The material was originally invented to make billiard balls, but never took off because the original formula was explosive. The dull side of a piece of unprocessed film is the emulsion side. It is also the business end of things. The emulsion is a gelatine mixture containing silver halide, the light-sensitive component of film. These days things are much more complicated than that, but let's start with simple black and white film, which is really just one layer of silver and nothing else. The silver halide clumps together into crystals in the emulsion. When light strikes a crystal it undergoes a change. Once the film has been exposed to light and is taken into the darkroom for processing, it is subject to a three-step process, the first step being to immerse it in developer. This chemical turns black those crystals that have been exposed to light. Timing in this process is critical because under-development will leave the exposed crystals unchanged, and over-development will blacken too much of the image. The second step is the stop bath, which arrests the development process. The third step is called the fix. In the fix, the unexposed silver halide crystals and other unneeded parts of the emulsion are washed away. The film is also rendered light-safe. Now you are left with an image in which parts that were exposed to light have turned black and parts that were not are transparent: a black and white negative. COLOUR NEGATIVE FILMS work in the same way, except three layers are formed within the emulsion rather than one. Each captures a different range of colours and each contains a dye that colours it. When you look at the negative, you see all three layers forming the colour image. Prints are pieces of film with a backing made of paper , not celluloid. They are produced when light is projected through the negative onto the paper. Because you are now making a negative image of a negative image, you end up with a positive. Making slide films involves an extra step - the reversal bath, which changes the negative image to a positive one. There are two types of colour slide - Kodachrome and everything else. Kodachrome is the original colour film that Kodak marketed in 1935 (colour negative film didn't come along until 1945). Before the late 1980s, it was the only film a respectable professional would consider using. It differs from every other type of colour film, positive or negative, in that the dyes that make up the colours in the film are added during the processing (with all other films, the dyes are already in the film when you buy them). Processing Kodachrome is not only complex but environmentally unfriendly, which is why Kodak is trying to phase it out.