more than seventy million people in the united states go to the movies each week to get away from their care and to find entertainment man through the magic screen but only if you understand  the lights and shadows on the screen are controlled to make a motion picture pictures are based on an optical illusion just another case where seeing is believing to understand how we see let's turn the tables on the human eye and look at it instead of through it the light reflected by any object we look at passes through the lens of the eye and is focused upside down on the sensitive service called the retina this rather than up passes the impression onto the optic nerve which sends it to the brain and we see it right side up the reflection will stay on the retina not as long as the object is in view when we look away or take the object away the image is disappear but notice that the image in the brain does not disappear immediately it's  a fraction of a second after the object is removed this slow fading is called first systems of vision when we look at a moving object we really see it in two places at once where it is and where it was a fraction of a second before let's get out into a rainstorm for a good example the water you seem to see falling in streams is really a lot of separate drops  make you see each rain drop in several places over a distance of several inches let's look at a picture of a car and a picture of a moving landscape to show how the eye can make them seem to overlap let's run them on the screen one after the other we look at them for two seconds each then for one second each now baster and faster until we see the two pictures as if they were both one picture vision is the principle which makes movies possible all motion picture is really only a fast series of still photographs that overlap in your mind three things of two of these still photographs placed one on top of the other show that each of the two photographs has caught the press in a slightly different position here is the way the first photograph looks this is the way the second photograph looks now let's take the complete series in rapid succession faster persistence of vision makes us see two pictures at once as a result there is an appearance of motion so all motion pictures are just still pictures joined together on a long strip of film the more still pictures we get while the action is going on the smoother it looks if we want to slow down the motion of something that goes fast we speed up our camera and make say five times as many pictures per second we have slow motion when they are shown on the screen at normal camera speed now here is a motion picture film a thousand feet sixteen thousand separate photographs let's tidy up this tangle of film by putting it on a real now we will build a projection machine to show the pictures on a screen the film on ones from the real everyone's onto another real after it runs through the projector so throw the picture on the screen there is a bright light with a reflector behind it to send the light rays in the right direction and the lens to concentrate the light the film passes or pull or after a tour which lets the light go through only one picture at a time there is another lens to focus the pictures on the screen by adjusting the lenses the picture can be made sharp and clear let's start the machine moving the still pictures blur because they're moving all the time and this won't do it all we need something to hold each picture still long enough for us to look at it in between the changes when the film is started and stopped in rapid succession the eye sees the changes being made and the result is streaks instead of movies so we need a shutter to cut off the light while every change of picture is being made and let the light through only while the film is standing still surprising as it seems you never see a motion picture while it is actually moving the only time a movie moves is when the shutter keeps you from seeing the change of picture now we have the important part of a project for showing motion pictures on the screen but let's change that drawing to an actual machine at work this real holes the supply of motion picture film which passes through the projector at a speed of ninety feet each minute slots on the separate your plate guides the film past the narrow beam of light so that only one picture can be seen at the time the adjustable limb which gives us a clear sharp picture on the screen is placed in the path of the light rays after they pass through the film sprockets which god the film as well as start it and stop it are placed in position to hold the film in proper position and now when you see motion pictures you get a true picture of what really happened up picture that you could never see in any other way