After the experiment at home in Kent, Darwin distributed a questionnaire around the world. And although he couldn't send the pictures themselves, he asked about facial expressions and gestures such as blushing and shrugging among different cultures.
His connections ensured that he received replies from travelers and missionaries as far as South Africa, India, China, North America, and Australia.
Darwin aimed to prove that there is a series of "cardinal" emotions that are expressed and perceived by all humans in the same way, and that these are innate or biological.
The study formed part of his 1872 book The Expression of the Emotions in Man and Animals in which he outlined his view that expression was a trait that humans shared with beasts.
Interestingly he also touched on the concept we now call psychophysiology - the study of the way in which your mind and body affect one-another writing: “The free expression by outward signs of an emotion intensifies it. On the other hand, the repression, as far as this is possible, of all outward signs softens our emotions.”
Sources: BBC and Scientific American