It may come as a surprise in the twenty-first century to discover that, in the 1880s, details of how objects move were unknown. The human eye, unaided, cannot resolve the details of fast motion. Eadweard Muybridge and his experiments with motion photography, such as this series of pictures of a horse's gait, helped solve this mystery.

From http://www.hrc.utexas.edu/exhibitions/permanent/windows/southeast/eadweard_muybridge.html - edited