Part 4

The plate holder is constructed to hold three dry plates, each three inches by twelve inches; the front is divided into twelve compartments, each three inches square.

Light is excluded from the front by a roller blind, strengthened by thin narrow slats of hard wood; the blind works in grooves, is drawn over a concealed roller, and covers the back of the holder when the plates are being exposed.

Figure 6 is a rear and side view of the circuit maker, conventionally called the exposing motor.

The motive power is an adjustable weight attached to a cord which is wound around a drum. Twenty-four binding posts are attached to the table at the back of the exposing motor; other binding posts are arranged for return or other currents.

Figure 7 illustrates a front and side view of the upper part of the exposing motor. Fastened to the frame is a ring of hard rubber, in which are inserted twenty-four insulated segments of platinum-coated brass; these segments are connected by insulated wires to the twenty-four binding posts on the back of the motor table, figure 6.

A shaft, connected by an arrangement of geared wheels to the drum, passes through the center of the segmented ring and carries a loose collar; a stout metal rod is firmly attached near its longitudinal center to this loose collar. One arm of the rod carries a laminated metal scraper, or contact brush, arranged to travel around the periphery of the ring, and in its revolution to make contact with each segment in succession. The contact brush is connected through the arm with one pole of the battery; and each segment--through its independent wire and magnet of the electro-exposors--with the other pole.

When twenty-four consecutive phases of an act of motion are to be photographed from one point of view, all of the insulated segments in the ring are put in circuit. When twelve consecutive phases are to be photographed synchronously from each of three points of view, each alternate segment is placed in circuit with the electric battery.

The manner in which the series of synchronous exposures is effected will be readily understood by reference to the diagram, 8.

All being in readiness, and the weights and fan wheel adjusted to cause the contact brush to sweep over the periphery of the ring at the required rate of speed, the drum, and with it the shaft is set in motion.

At the proper time, pressure on a button completes an independent circuit through the magnet seen below the segmented ring, figure 7, and in the side diagram of figure 8.

The action of the armature releases the lower end of the rod on the loose collar, which, by means of a coiled spring, is immediately thrown into gearing with the already revolving shaft; the contact brush sweeps around the segmented ring and effects the consecutive series of exposures at the pre-arranged intervals of time.

At the University the intervals varied from the one-sixtieth part of a second to several seconds.

A record of these time intervals was kept by a chronograph, a well known instrument; it comprises a revolving drum carrying a cylinder of smoke-blackened paper, on which, by means of successive electric contacts, a pencil is caused to record the vibrations of a tuning fork, while a second pencil marks the commencement of each photographic exposure. The number of vibrations occurring between any two successive exposures marks the time. The tuning fork used made one hundred single vibrations in a second of time. To ensure greater minuteness and accuracy in the record, the vibrations were divided into tenths, and the intervals calculated in thousandths of a second.

For the purpose of determining the synchronous action of the electro-exposors while making a double series of exposures, the accuracy of the time intervals as recorded by the chronograph, and the duration of the shortest photographic exposures used in the investigation, the two batteries of portable cameras were placed side by side, and the exposors were each connected with the exposing motor by separate lengths of a hundred feet of cable. The two series of cameras were pointed to a rapidly revolving disc of five feet diameter. The surface of the disc was black, with narrow white lines radiating from the center to the edge like the spokes of a wheel. A microscopic examination of the two series of resulting negatives proved that no variation could be discovered in the synchronous action of ten of the duplicated series of exposures, and that in the remaining two a variation existed in the simultaneity of a few ten-thousandths of a second--a result sufficiently near to simultaneity for all ordinary photographic work.

A reproduction of the chronographic record of one of these experiments is seen in figure 9.

The first line records the revolution of the disc; the second the vibration of the tuning fork; and each group of three long double markings in the third line indicates a photographic exposure.

The shortest exposures made at the University were--approximately--the one six-thousandth part of a second; such brief exposures are however for this class of investigation very rarely needed.

Some horses galloping at full speed will, for a short distance, cover about fifty-six or fifty-eight feet of ground in a second of time; a full mile averaging perhaps a hundred seconds. At this speed, a foot recovering its loss of motion will be thrust forward with an occasional velocity of at least 120 lineal feet in a second of time.

During the one one-thousandth part of a second the body of the horse will at this rate move forward about seven one-tenths of an inch, and a moving foot perhaps one and a half inches, not a very serious matter for the usual requirements of the amateur photographer.

A knowledge of the duration of the exposures, however, was in this investigation of no value, and scarcely a matter of curiosity, the aim always being to give as long an exposure as the rapidity of the action would permit, with a due regard to the necessary sharpness of outline, and essential distinctness of detail.

The power used for operating the magnets, through the exposing motor, was given from a lé Clanché battery of fifty-four cells, arranged in multiple arc of three series, each of eighteen cells.

During the investigation at the University of Pennsylvania, more than a hundred thousand photographic exposures were made.

The negative plates were supplied by the Cramer Dry Plate Company of St. Louis, and the positive plates by the Carbutt Company of Philadelphia. On a favorable day five hundred or six hundred negatives were sometimes exposed; on one day the number of exposures reached seven hundred and fifty.

The electrical manipulations were directed by Lino F. Rondinella; the development room was in charge of Henry Bell. The author takes pleasure in acknowledging the skill, patience and energy which these gentlemen exhibited in their respective fields of labor.

Although the one six-thousandth part of a second was the duration of the most rapid exposure made in this investigation, it is by no means the limit of mechanically effected photographic exposures, nor does the one-sixtieth part of a second approach the limit of time intervals. Marey, in his remarkable physiological investigations, has recently made successive exposures with far less intervals of time; and the author has devised, and when a relaxation of the demands upon his time permit, will use an apparatus which will photograph twenty consecutive phases of a single vibration of the wing of an insect; even assuming as correct a quotation from Nicholson's Journal by Pettigrew in his work on Animal Locomotion that a common house fly will make during flight seven hundred and fifty vibrations of its wings in a second of time, a number probably far in excess of the reality.

The ingenious gentlemen who are persistently endeavoring to overcome the obstacles in the construction of an apparatus for aerial navigation, will perhaps some day be awakened by the fact that the only successful method of propulsion will be found in the action of the wing of an insect.

We will now resume the subject proper of this monograph.

It is impossible within its limits to trace the history of the art of delineating animals in motion, or to illustrate it with examples of the truthful impressions of the primitive Artists, or of the imaginative and erroneous conceptions of many of those of modern times. Certain phases of the facts of Animal Locomotion will alone be treated upon, as demonstrated by photographic research.