Dear Dan: Here in town we have a funny little store called the Beadshop. The bead that I used was of solid brass with a tiny 1/32 inch hole clear through. I passed a thin brass wire through the hole and cemented it there with epoxy. I fixed the wire to brass nuts and bolts which were in the 1/16 inch thick (#14 grade) brass plate through which I drilled the 1/4 inch hole, being careful to center the bead exactly.
John Carmichael Tucson >>Hello dialists: >> >>Three years ago I built an equitorial interactive mechanical >>heliochronometer of brass and wood based on the design described in chapter >>xII, pgs. 193-202 of the Mayall's book. "The heliochronometer consists of >>four basic parts: base, dial plate, alidade or sighting instrument, and >>analemma. The alidade is attached to the dial plate so that it can be >>rotated about its center, which is coincident with the center of the dial >>plate. Consisting of a flat plate, the alidade has two fixed upright arms >>perpendicular to the dial plate. One arm contains the style or nodus, the >>other the analemma." The Mayall's suggest that the style or nodus may be >>either a simple pinhole (a shadow sharpener), the intersection of two >>crosshairs, or a bead centered inside of a small hole. They didn't say >>which type is better, however. >> >>To determine this, because I didn't know the necessary optical mathematics, >>I conducted over thirty different experiments using all sorts of hole, >>crosshair and bead diameters. The objective, of course, was to find the >>style which cast the smallest point of light or shadow onto the analemma. >> >>The design which worked the best was a 1/8 inch spherical bead, suspended by >>thin brass crosswires, in the exact center of a 1/4 inch round hole. (The >>style was about 24 inches from the analemma). >> >>A very curious thing happens with this type of style. The bead alone, by >>itself, casts a shadow that was twice as big as the bead; 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, smaller than the bead itself!!!! The wires which keep the bead >>suspended in the middle of the hole are so thin that they don't cast a >>visible shadow onto the analemma. >> >>This arrangement somehow has the ability to sharpen the shadow of the bead. >>I don't know how this works, but it does. It probably has something to do >>with the wavelength of light or diffraction. My experiments showed that >>this effect only worked for a style with these dimensions; larger or smaller >>beads, holes or gaps did not exhibit this strange focusing phenomena. >> >>I don't know why this works, but it does. Can any of you explain this? >> >>John Carmichael >> >>p.s. I believe I sent photos of this style on my heliochrometer to several >>of you to whom I sent copies of my manual (Roger, Ross, Susan, Harold, Fred?) >> >> >> > >John > >I wonder if you could tell me how you suspended the bead with the wires? Seems >like a good bit of fine construction. What was the bead made off? I am >tempted to make >such an item to use with my dial. > >Thank you. Dan Wenger > >Daniel Lee Wenger >Santa Cruz, CA >[EMAIL PROTECTED] >http://wengersundial.com >http://wengersundial.com/wengerfamily > > >
