Complex sentence: Originally a photographic slide made up of two transparencies in perfect registration--one containing the colour information and the other the monochrome background--the card was converted to electronic form in 1984 when electronic storage became possible.
Split into simpler sentences: Originally a photographic slide made up of two transparencies in perfect registration--one containing the colour information and the other the monochrome background. The card was converted to electronic form in 1984 when electronic storage became possible.