n the time of passage of the space between the
slits, the object had moved by a certain amount across the field of
view, a fresh impression was produced at each exposure. The object,
well illuminated by sunlight, moved in front of a black background.
Since the angular velocity of the disk was known, and the number of
slits, the time between the successive positions of the object was
also known.

Marey (_La Methode graphique_, pp. 133, 142, 456), by means of
pneumatic signals and a rotating cylinder covered with smoked glazed
paper, measured the time of the movements of the limbs of animals. The
instrument consists of a recording cylinder rotated at a uniform
angular velocity by clockwork controlled by a fan governor, and
pneumatic signal, constructed thus. One end of a closed shallow
cylinder, about 4 cm. dia., is furnished with a stretched rubber
membrane. A light lever, moving about an axis near the edge of the
cylinder, is attached to the centre of the membrane by a short rod,
its free end moving as the membrane is distended. The cylinder is
connected by a flexible tube with a similar cylinder and membrane, but
without a lever, which is attached to that part of the body of the
animal the movement of which is under investigation. The system is
full of air, so that when the membrane attached to the animal is
compressed, the membrane which moves the lever is distended and the
lever moved. Its end, which carries a scribing point, marks the smoked
paper on the rotating cylinder. The pneumatic signal is called by
Marey "tambour a levier."

_References to Chronographic Methods:_--(1) Chronographs used in
Physiology: Helmholtz, "On Methods of measuring very small Portions of
Time," _Phil. Mag._ (1853), 6; Id., _Verhandlungen der
physikalisch-medicinischen Gesellschaft in Wuerzburg_ (1872); Harless,
"Das Attwood'sche Myographion," _Abhandlungen der k. bayerischen
Akademie der Wissenschaften_ (1862); Id., _Fall-Myographion
aufgestellt in der Wiener Weltausstellung in der Abteilung fuer das
Unterrichtswesen von Ungarn_ (Budapest, 1873); Hensen, "Myographion
mit vibratorischer Bewegung," _Arbeiten aus dem Kieler physiol.
Instit._ (1868); Bruecke, _Sitzungsber. d Wien. Acad._ (1877); Pflueger,
"Myographion ohne Bewegung," _Untersuchungen ueber die Physiologie des
Electrotonus_ (1859); Pouillet, _Compt. rend._ (1844); I. Munk,
_Physiologie des Menschen_ (f