The Horse in Motion 1878 - Silkscreened

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In 1872, the former governor of California Leland Stanford, a businessman and race-horse owner, hired Muybridge for some photographic studies. He had taken a position on a popularly-debated question of the day — whether all four feet of a horse were off the ground at the same time while trotting. The same question had arisen about the actions of horses during a gallop. The human eye could not break down the action at the quick gaits of the trot and gallop. Up until this time, most artists painted horses at a trot with one foot always on the ground; and at a full gallop with the front legs extended forward and the hind legs extended to the rear, and all feet off the ground.[26] Stanford sided with the assertion of "unsupported transit" in the trot and gallop, and decided to have it proven scientifically.

Stanford sought out Muybridge and hired him to settle the question. In 1872, Muybridge settled Stanford's question with a single photographic negative showing his Standardbred trotting horse Occident airborne at the trot. This negative was lost, but the image survives through woodcuts made at the time (the technology for printed reproductions of photographs was still being developed). He later did additional studies, as well as improving his camera for quicker shutter speed and faster film emulsions. By 1878, spurred on by Stanford to expand the experiments, Muybridge had successfully photographed a horse at a trot;[28] lantern slides have survived of this later work.[29] Scientific American was among the publications at the time that carried reports of Muybridge's groundbreaking images.

Stanford also wanted a study of the horse at a gallop. Muybridge planned to take a series of photos on June 19, 1878 at Stanford's Palo Alto Stock Farm. He placed numerous large glass-plate cameras in a line along the edge of the track; the shutter of each was triggered by a thread as the horse passed. The path was lined with cloth sheets to reflect as much light as possible. (In later studies he used a clockwork device to set off the shutters and capture the images.) He copied the images in the form of silhouettes onto a disc to be viewed in a machine he had invented, which he called a zoopraxiscope. This device was later regarded as an early movie projector, and the process as an intermediate stage toward motion pictures or cinematography.

The study is called Sallie Gardner at a Gallop or The Horse in Motion; it shows images of the horse with all feet off the ground. This did not take place when the horse's legs were extended to the front and back, as imagined by contemporary illustrators, but when its legs were collected beneath its body as it switched from "pulling" with the front legs to "pushing" with the back legs..