Evening Republican, Volume 17, Number 284, Rensselaer, Jasper County, 28 November 1913 — Illusions of a Passing Car [ARTICLE+ILLUSTRATION]

Illusions of a Passing Car

Why a r Wheel Sometimes See mi to Revolve Backward-

Under this heading, in The Autocar. Charles F. Street notes and explains the fact that at night a moving wheel frequently appears to revolve abnormally, at times going too Slowly, again appearing to stop, and still again seeming to turn backward. Under similar conditions, if a walking stick be twirled in the hand, a series of distinct images will be observed at intervals, their distance apart depending upon the speed of whirling. We read: “These illusions, are only obtainable In the light emitted from an electric lamp supplied with an alternating current. The alternating current changes its direction, in some cases 40 times per second, and in others as many as .90 times per second. “In order that the current may change its direction, there must be an instant In every reversal when there Is a momentary but entire cessation of the current, and this cessation necessarily affects the emitted light In the form of a very short flicker. “Owing to a peculiar property of the human eye, these cessations of the light, when the eye is viewing any fixed ' object, are not noticeable. • * • "In the case of the moving walkingstick, an impression is gained as to the position of the stick at an instant of maximum illumination. This impression is retained by the eye, and the various positions occupied by the stick during the period of less brigh. light. are not appreciated. * • • "Suppose that a wheel is under observation, and that an Impression is formed in accordance with the posi-

tlon of the spokes, and that the view Is then obstructed for r time, during which period of obstruction th@ wheel, having ten spokes, say is turned through one-tenth of a' revolution. “If the obstruction referred to be removed, and another observation of the wheel be made, there would be absolutely no evidence that the wheel had been turned at all, the sppkeg being all alike in appearance, and now occupying a similar position as before. ' —...

“This 1b exactly what happens when a wheel Is viewed in an artifleal light supplied with alternating current * * * Therefore if the speed of a wheel be such that the spokes occupy the same relative position to the car every time the light reaches its maximum value, the sen" sation gained with regard to the movement of the wheej. is that it is not revolving at all, and this speed can be defined as synchronous speed." If the wheel Is revolving at a speed slightly less than synchronous, then the first impression of the spokes is as shown In the figure by the full lines, and the next is obtained when the

spokes are in some position as shown dotted behind the first- The wheel thus appears to revolve backward — the Jumps from one position to the other being unnoticed. If the speed t>e greater than synchronous, these spokes appear a little further ahead each time, and the wheel appears to be moving slowly forward. "Another illusion is obtained by watching the wheels of a passing car through a length of distance of, say, ten yards from the observer. Such a slew is almost unique in demonstrating the well-known fact that the top of a rolling wheel moves at twice the speed of the center of the wheel, and Infinitely faster than the bottom of the wheel, both relative to the earth."

Successive Images That Make a Wheel Seem to Revolve Backward.