Message text written by John Carmichael >but when the 1/8th in. bead is in the center of a 1/4" hole, with a space of 1/16th of an inch between the bead's edge and the hole edge, the bead's shadow miraculously sharpens into a tight, dark shadow that is only 1/16th of an inch in diameter,<
It is simply the pin hole camera effect again. Light passing through any small aperture is focused . As the hole's size is changed the focusing parameters are changed too. So with a fixed distance from hole to plate there will be one size that works. A different size would be needed for different image distances. The effect was originally made known - though probably not discovered - by Porta in the middle of the 16th century. He found that if a small hole was made in the shutters of a room an inverted image of the objects outside in their true colours was thrown onto the opposite wall. The hole doesn't have to be round - indeed you can see the effect by placing your eyes very close (3 inches or so) from a pattern - so that you cannot focus on it - and half closing your eye lids. At some point the pattern will come into focus. Incidentally it will also appear to move as you do so. This is the effect you see too. What I find interesting about the effect is whether or not it was used in the design of meridian lines where a spot of light crossing a marked meridian line was used to establish all sorts of astronomical parameters. I suspect the variation in image distance was too great. Patrick
