Hi Bill, You must be right. Earlier, I did not dare to suppose that Wheatstone, a famous scientist, and William Darker, a valued instrument maker, could err so clearly, but it's the only explanation left.
Maybe a confusion arose from the report of Wheatstone's presentation in the Association Meeting in Swansea, August 1848. The report goes (thanks for the link, Patrick Vyvyan): "On a plate of glass twenty-five films of selenite of equal thickness are arranged at equal distances radially in a semicircle [...] and figures corresponding to the hours are painted above each film in regular order. [...] the hour is indicated by the figure placed opposite the radius which contains the most red; the half-hour is indicated by the equality of two adjacent tints." The "25 films of selenite" would clearly have enabled to identify full and half hours by the film that "contains the most red", and quarter hours by "two adjacent tints". However, the description suggests to paint 25 HOUR numbers in the semicircle, as we see done today. The description in the RMG database says that "an instrument of this type was exhibited at the 1848 meeting". I doubt whether that is true; at least it cannot be read from the report. Most of the text is devoted to another polarizing sundial type, of which also no mention of an exhibition is made. The present type is only described in the last paragraph of the report as one of "several other forms of the polar clock". Thanks to all who responded! Best regards, Frans Maes <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> Virusvrij. www.avg.com <http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> On Wed, May 22, 2019 at 4:04 PM John Davis <[email protected]> wrote: > Hi Frans et al, > > Polarizing sundials have featured a number of times in the *BSS Bulletin*. > In the same issue (21(i), March 2009) as the article by the sadly-missed > Allan Mills that Mike Isaacs pointed to earlier, Allan had a second > article, pp. 14-16, on "An Electronic Polarization Sundial and Photometer" > which shows background experiments on the physics that Wheatstone's device > is based on. The first of these articles was one of the earliest we printed > in colour, essential to see the proper effect of Allan's dial. > > Earlier articles in the *Bulletin* are by Allan Mills again ("The > Sellotape Sundial" 98(1) 3-9) and by David Colchester "A polarized light > sundial" 96(3) 13-15. > > I think reading these articles will fully explain Wheatstone's device, of > which several were evidently made. > > Regards, > > John > ---------------------------------- > > Dr J Davis > Flowton Dials http://www.flowton-dials.co.uk/ > BSS Editor http://sundialsoc.org.uk/publications/the-bss-bulletin/ > <http://www.sundialsoc.org.uk/bulletin.php> > > > > On Tuesday, 21 May 2019, 21:06:52 BST, Maes, F.W. <[email protected]> > wrote: > > > Recently the Tesseract Catalogue 109 was announced on this list. Item nr. > 13 is a polarizing sundial by Charles Wheatstone. A virtually identical > dial is in the collection of the Greenwich museums, see: > https://collections.rmg.co.uk/collections/objects/265579.html. > I have never understood how this sundial works. The hour scale shows 2 x > 12 hour numbers in a semicircle. So whatever pattern is observed in the > black glass reflector, it is obviously supposed to rotate over 180° in 24 > hours, which is half the angular velocity of the sun itself. How does this > frequency division-by-two come out? Can anybody explain? > > Thanks! > Frans Maes > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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