Magic, Stage Illusions and Scientific Diversions, Including Trick Photography

by Albert A. Hopkins

FIG. 7.—INTERIOR ARRANGEMENT OF THE DARK CHAMBER.

[Fig. 3] shows the parabolic trajectory of a brilliant ball thrown across the face of a dark screen; but it is discontinuous, as exposures were only produced each fiftieth of a second on account of the number of the openings and the speed of the rotation of the disk. This is only an example which shows the almost limitless number of varieties of movement which may be analyzed by chronophotography.

With ordinary shutters it would be difficult to obtain this quickness, but the perforated disk which is used in chronophotography gradually acquires a speed of rotation that may be very great. [Fig. 4] shows the arrangement of this disk by which a rotary movement is imparted by a powerful gearing controlled by a regulator. As soon as the disk obtains a speed of ten turns a second, the regulator maintains this speed with perfect uniformity. The disk moves in front of the sensitized plate a few millimeters only; then, knowing the angular value of each of the openings, the period of exposure is easily deduced therefrom.

FIG. 8.—WALKING MAN, CLOTHED IN WHITE,
PASSING ACROSS THE FIELD.

The condition most difficult of fulfillment is the absolute darkness of the screen before which the photographs are taken. Little light as there is, the screen might reflect upon this sensitized plate, during a single exposure, small quantities of light, which would tend to fog the plate. A wall painted with any black pigment, or even covered with black velvet, exposed to the sun, reflects too much light for a plate to withstand. The term “black screen” is used in a metaphorical sense. In reality the work is done before a dark cavity, being in truth what is known as “Chevreul’s black.” To obtain these favorable conditions, a chamber nearly thirty-three feet deep and of equal breadth was constructed; one face of this chamber was open, and restricted by movable frames to the exact height necessary. The interior of the chamber was completely blackened, the ground was coated with pitch, and the back hung with black velvet.