Behold! human beings living in an underground den,which has a mouth open towards the light and reach-ing all along the den; here they have been from theirchildhood, and have their legs and necks chained sothat they cannot move, and can only see before them,being prevented by the chains from turning roundtheir heads. Above and behind them a fire is blazingat a distance, and between the fire and the prison-ers there is a raised way; and you will see, if you look,a low wall built along the way, like the screen whichmarionette players have in front of them, over whichthey show the puppets.I see.And do you see, I said, men passing along the wallcarrying all sorts of vessels, and statues and figuresof animals made of wood and stone and various ma-terials, which appear over the wall? Some of them aretalking, others silent.You have shown me a strange image, and they arestrange prisoners(Plato, Republic, Book VII).
Scan line - left to right, top to bottom, 525 lines perscreen (NTSC) or 625 lines (PAL), two interlaced scans(odd and even lines) per image, 25 frames per secondHorizontal retrace: after each scan line the electrongun is switched off and returned to the beginning ofthe next lineVertical retrace: after each field has been scanned, theelectron gun returns to the top left of the screen tobegin the next
Diagram of a mirror-lens condenser used in motion-picture projection: (S)light source (electric arc), (M) parabolic mirror, (L) lens, (G) film gate