One of the very first questions posed on this list was >>Does anyone know about dials... that use actual lens type optical systems? An updated version of my response follows. Unfortunately, there was no further discussion of this interesting topic.
A dial that uses optics would need to track the sun. A possible exception would be a spherical glass element that projected an image of the sun onto a concave surface. Does a sundial by definition have a fixed dial and a moving shadow? Does an implement that tracks the sun, has a 'fixed' shadow, and 'tells the time' with a moving indicator on some scale, qualify as a sundial? Or is it then a sextant? Sundials with optical elements could have at least two functions: to focus the sun's rays and provide a more collimated beam of light, and to allow a pointing accuracy far greater than the naked eye allows. An ingenious designer could use a concave mirror for these purposes, but using a lens would be a simpler construction. The scarcity of this type of dial could be because optical elements do not weather well, or the complexity of a tracking mechanism, or because the accuracy of a sundial is limited by several considerations. At some point, increasing the accuracy of a dial runs into the inaccuracies imposed by the sun and atmosphere. One limit is placed by atmospheric refraction, which varies with the height of the sun in the sky, the altitude of the observer, and atmospheric conditions of pressure, temperature, and humidity; as detailed in the North American Sundial Society Compendium of 12/95. The diameter of the sun and the brightness of the sky cause any shadow to be indistinct, discussed in the Compendium of 8/94. A shadow cast by the sun has an indistinct edge, because of the one-half degree diameter of the sun. Using optics, an image of the disk of the sun could be projected or observed, with a very sharp edge to allow precise measuring. Finally, irregularities in the rotation of the earth place a limit on the accuracy of a sundial, probably to an infitesimal degree A discussion of the theoretical limits of accuracy of a dial would be of great interest, and in fact was mentioned as a sequel to the 8/94 article, leaving me on the edge of my chair for some time now. Other aspects of optical sundials are equally fascinating. Christopher Schissler's 1578 dial, now at the American Philosophical Society in Philadelphia, was the subject of an article in the Feb. 10, 1990 Science News. This is a bowl sundial with an ellipsoidal inner surface, calibrated for two regions of latitude. When filled with water, the shadow of the gnomon (a bead on a string) is refracted to indicate one hour previous to the unrefracted shadow, a simulation of the story in Isaiah 38. Another optical dial is the polar clock of Charles Wheatstone, where the approximate time was indicated by the polarization angle of light from the sky. Another is the dipleidoscope, detailed in Gerard Le Turner's Nineteenth Century Scientific Instruments This meridian instrument held English patent 9793, June 20, 1843, by James Bloxam. It had a hollow 90 degree prism, and when the image of the sun reflected off the top glass coincided with the image from the two silvered sides, noon had occurred within a few seconds. One of the depicted models includes an optical sighting tube. Those who yearn for greater accuracy from sundial type instruments might well consider the coincidence of two reflected images. As with a vernier micrometer, exact coincidence of two lines or edges can be established with great precision, and it is easier to do than measuring with a scaled ruler. The sextant uses the coincidence of an image of the sun and the horizon. In addition there is the simple ceiling dial, and no doubt there have been many variations on the theme of a dial inside or in the shade, using a mirror to reflect the sunlight. Are there other dials that refract or reflect sunlight, and for what purpose? Peter Abrahams, [EMAIL PROTECTED] \\\\\\\\\\\\\\\\\\\\//////////////////// Peter Abrahams [EMAIL PROTECTED] the history of the telescope, the microscope, and the prism binocular
