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?)
