Analysis of information sources in references of the Wikipedia article "The Expression of the Emotions in Man and Animals" in Simple English language version.
Heliotype was a new photomechanical method of reproduction invented by the photographer Ernest Edwards (1837–1903), for whom Darwin had sat for a portrait in 1868. Although he had no experience in photographic publishing, Darwin suggested this new technique to John Murray. ... heliotype reduced the cost of production considerably, enabling Darwin to afford the number of photographs appearing in Expression.
