>Does anyone know about dials... that use actual lens type optical systems?
Sundials with optical elements could have at least two functions: to focus
the sun's rays and provide a narrower 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 because the accuracy of a sundial is
limited by several considerations. 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.
Finally, irregularities in the rotation of the earth place a limit on the
accuracy of a sundial, prehaps to an infitesimal degree; has this topic been
addressed in the literature? 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.
The previous post on the dipleidoscope is detailed in Gerard Le Turner's
Nineteenth Century Scientific Instruments (for those without domestic sundial
museums.) 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. The dimmed image of
the sun, reflected or refracted, gives a sharp disc to measure. Prehaps the
next
millenium will give us the first Hydrogen-alpha solar prominence sundial.
Other examples will be the subject of future posts, by which time I will
hopefully have found a terminology to describe sundials that use focused
light, since all
sundials can be described as optical instruments. --'Peter Abrahams
[EMAIL PROTECTED] (Peter Abrahams)
the history of the telescope,
the prism binocular, and the microscope
