It was James Clerk Maxwell who first showed that by taking three separate pictures, each through a filter (red, blue and green) and by superimposing the lantern slides (again with the appropriate filter in each projector), colours could be re- created. The three negatives produced in this way are called colour separation negatives, because the visible spectrum has been separated into three parts.
Early attempts to reproduce colour began at the turn of the century, but were out of the reach of all but the more dedicated and wealthy photographer. Initially the subjects were still-life ones (obviously the three images had to be exactly the same), though cameras were then produced which exposed three pictures simultaneously.
© Robert Leggat, 1999.