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��Popular Science Monthly

��Why Wheels Run Backward on the Motion Picture Screen

WHEN the moving-picture camera takes a picture the film is not exposed continuously. Instead it is uncovered and then covered again in very rapid succession by a black disk edged with a circle of holes which are swiftly rotated across the front of the lens. In this way a series of pictures is taken which represents the successive movements of the subject that is photographed. But because of this very fact, the speed at which an object appears to rotate when the developed film is projected on the

���case of this kind, if the spoke and the camera keep turning at constant speeds, when the spoke is projected on the screen it will appear to be standing perfectly still. Again, the wheels on fast moving cars often appear as if they were moving backward while the car is going for- ward. This would happen if the spoke A , shown in the bottom row of the drawings, had been revolv- ing so fast when it was photographed, that during the interval between one exposure and the next, the wheel revolved all the way around from A to B. When the next

��The speed of rotation of the wheels may be such as to make it appear to stand still as in the top row, or even to be running backward as in the bottom row

��screen is very seldom the speed at which it actually rotated.

Take, for instance, a four-spoked wheel, such as shown in the illustration, where the wheel rotates at a speed such that after one picture of the film is exposed, the spoke A has turned around just enough to show A at the position of B when the next pic- ture is exposed. It is evident that in a

��exposure was made the spoke appeared at C, and so on. When this film is run off and projected on the screen, the spoke will seem to run backward. No matter how many spokes there are on the wheel it is evident that the effect on each one will be the same, and the 'entire wheel will appear to turn backward at the same speed as that of each spoke. It is merely an optical illusion.

��Money Prizes for Motorcyclists

��IF you are a motorcyclist, if you have devised simple ways of making repairs, if you have improved your machine in any way, this will interest you.

The Popular Science Monthly offers a first prize of \$25, a second prize of \$15 and a third prize of \$10 for articles in which motorcyclists will describe and illustrate the methods which they have successfully employed for overcoming trouble, for making quick repairs by the road- side or more difficult repairs in the shop, or for making attachments whereby the use of the motorcycle has been broadened.

The three prizes will be awarded by the editors of the Popular Science Monthly in the order of merit. What is more, even though your article may not win a prize, the editors may buy it at the usual rates, just because it is so good.

There are no limitations to this prize offer. We don't care for fine phrasing, but we do care for good mechanical ideas. Rough pencil draw-

��ings or photographs will do for illustrations. The following conditions are to be observed:

(1) Articles must be written on one side of the sheet

only.

hand corner of the first sheet.

(3) Enclose postage for the return of the manuscript.

(4) Don't send in articles on ideas which have already

been published.

(5) Don't send paper ideas — things that you haven't

actually done yourself.

(6) Address the envelopes containing articles to

"MOTORCYCLE CONTEST EDITOR"

Popular Science Monthly

239 Fourth Ave., New York City

The contest will close on December 31st, 1916. The money for the prizes will be paid promptly after the awards have been made.

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