That evening a great audience in the dark interior of a New York theatre sat watching a white sheet stretched across the stage; suddenly its white expanse grew dark, and against the background appeared "The Suburban, run this afternoon at 4:45 at Sheepshead Bay track; won by Alcedo, in 2 minutes 5 3-5 seconds." [Illustration: BIOGRAPH PICTURE OF A MILITARY HAZING SCENE These pictures are not consecutive. The difference between those that follow each other is so slight as to be almost imperceptible because of the rapidity with which they are taken. These pictures were probably taken at the rate of thirty to forty per second.] Then appeared on the screen the picture of the scene that the thousands had travelled far to see that same afternoon. There were the wide, smooth track, the tower-like judges' stand, the oval turf of the inner field, and as the audience looked the starter moved his arm, and the rank of horses, life-size and quivering with excitement, shot forth. From beginning to end the great struggle was shown to the people seated comfortably in the city playhouse, several miles from the track where the race was run, just two hours and fifteen minutes after the winning horse dashed past the judges' stand. Every detail was reproduced; every movement of horses and jockeys, even the clouds of dust that rose from the hoof-beats, appeared clearly on the screen. And the audience rose gradually to their feet, straining forward to catch every movement, thrilled with excitement as were the mighty crowds at the actual race. To produce the effect that made the people in the theatre forget their surroundings and feel as if they were actually overlooking the race-track itself, about five thousand separate photographs were shown. It was discovered long ago that if a series of pictures, each of which showed a difference in the position of the legs of a man running, for instance, was passed quickly before the eye so that the space between the pictures would be screened, the figure would apparently move. The eyes retain the image they see for a fraction of a second, and if a new image carrying the movement a little farther along is presented in the same place, the eyes are deceived so that the object apparently actually moves. An ingenious toy called the zoltrope, which was based on this optical illusion, was made long before Edison invented the vitascope, Herman Caster the biograph and mutoscope, or the Lumiere brothers in France devised the cinematograph. All these different moving-picture machines work on the same principle, differing only in their mechanism.