These are cue marks, or "reel-change dots," signaling the projectionistthat it is time to change reels. There is actually a set of dots.Four consecutive frames are marked with a little circle in the upperright-hand corner of the frame. The first set (4 frames) of cue marks(the motor cue) is placed 198 frames before the end of the reel. (198frames is 8.25 seconds, or 12.375 feet.) There are 172 frames betweenthe first set of cue marks and the second set of 4 frames, thechangeover cue. There are 18 frames between the changeover cue and therunout section of the trailer (or foot) leader. The projectionistthreads up the next reel of film so that he has about nine feet ofleader between the lens and the start of the film. At the first cuemark, he starts the motor on the second projector. This gives theprojector time to get up to to speed and for the speed to stabilize.On the second cue mark, he throws the switches that change the pictureand sound sources. In some old films on TV, you'll see long changeovercues since some projectionists were paranoid that they would not seethe marks.
Information was taken from: http://stason.org/TULARC/movies/current-films/10-What-are-those-funny-dots-that-blink-on-in-the-upper-rig.html
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