Thomas Eakins, recognized as the leading proponent of Realism in nineteenth-century American painting, often painted portraits of friends who were accomplished in the fields of science, medicine, and academia. William Dennis Marks, a St. Louis native and prominent professor of dynamical engineering at the University of Pennsylvania, is depicted here seated at his desk, seemingly lost in thought, surrounded by the books and tools of his profession. While the background is loosely painted, sharper focus is given to the engineer’s figure and the machine on his desk, a chronograph invented by Marks for use in experiments with motion photography. To paint it with precision Eakins transferred the outline of the machine from a photograph onto the canvas. The light shining on Marks’s head, his hands, and the chronograph visually unites these elements, suggesting a link between the engineer’s thoughts and his experiments. One of Eakins’s most ambitious portraits, this work demonstrates how the artist’s interest in technology and science was connected to his advancement of realism in painting.