The function of a developer is to convert the invisible (latent) image into a visible form. In the very earliest days of photography exposures had to continue until an image had been formed, hence the need for very long exposures.
It is said that Daguerre discovered development accidentally, having placed some unsuccessful sensitised plates into a cupboard, and having returned later to discover that the plates bore an image. By further experimentation he concluded that the mercury in the same cupboard was responsible for producing the image.
Fox Talbot also discovered that there was such a thing as a latent image that could be developed, and as a result was able to reduce drastically his exposure time - from as much as an hour to a minute or less.
© Robert Leggat, 1999.