With the assistance of Paul Pretsch, who invented the process, De la Rue succeeded in producing a copperplate for printing from a photographic negative of the moon, by using the unusual swelling properties of gelatin after it is exposed to light. De la Rue called the print a heliotype; we would today call this a photogalvanograph, since the gelatin was electroplated after exposure and treatment. By the end of the century, gelatin-based plates would allow exquisite reproductions of the lunar surface, in the form of photogravures and collotypes.