That is an excellent question! I have seen this photo before, and never noticed the numbers running twice in a semicircle. I, too, am perplexed. I read about this dial in Hester Higton's book "Sundials at Greenwich." The dial operates on two successive polarizations of light - the first being when light passes through the selenite strips on the glass, and the second when light reflects at the "the polarizing angle" (Brewster's angle?) off of the inclined dark glass plate behind the front glass. "At all times of the day the radii will appear of various shades of two complementary colours." This is different than how a single piece of polarized film would be used today.
Does this help anyone figure this out? -Bill On Mon, Mar 30, 2015 at 3:48 PM, Maes, F.W. <f.w.m...@rug.nl> wrote: > Hi all, > > Wheatstone designed still another type of polarization dial than the one > described by Jim Mahaffey. A specimen is in the collection of the British > National Maritime Museum in Greenwich, see > http://collections.rmg.co.uk/collections/objects/265579.html. > When viewing the celestial pole, the polarization pattern of the sky is > visible, consisting of two light and two dark regions, which rotate around > the pole together with the sun. > What I don't understand from the NMM dial: why does it have twice the hour > numbers from 1-12 in a semicircle, while the sun rotates through 24 hours > in a full circle? > Allan Mills made a modern version, using sellotape instead of selenite; > see BSS Bulletin 1998 nr. 1. It has one set of 1-12 hour numbers in a > semicircle, as one would expect. > > Best regards, > Frans Maes >
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