Jules Janssen developed a large photographic revolver to document the stages of the transit of Venus in 1874, regarded as an important method to determine the Astronomical Unit (the distance between earth and the sun). Several copies of the device were used at different geographic points. Many negative images of the different phases of Venus passing in front of the sun were captured on glass discs, in an early form of time-lapse photography. Unfortunately, the quality was not sufficient for a calculation of the Astronomical Unit. Several photographic revolver discs with images have been preserved, but research concluded that all of the known discs contained test recordings of a model in front of a circular light source (or brightly lit surface). Although the photographs were most likely never intended to be presented as motion pictures, much later images of one disc were transferred and animated into a very short stop motion film. No original footage of the photographic revolver recordings of the actual Venus transit has yet resurfaced. In 1875 and 1876, Janssen suggested that the revolver could also be used to document animal locomotion, especially birds since they would be hard to photograph by other means.