A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Geoffrey Kolbe
Date: 2021 Jun 5, 10:53 -0700
Peter Monta wrote;
"Aha, this might be a case of parallel invention then."
How interesting. I had never heard of a "Dipleidoscope". My apparatus is similar in the way it works to the Bloxham instrument, but uses a roof prism rather than a 60 degree prism. The front face of the roof prism has a dichroic coating, so that blue light is reflected off the front face in the manner of a normal mirror, but red light is reflected internally within the prism and - as far as azimuth angle is concerned - is "reflected" back at the sun.
The red "reflection" follows the sun in azimuth as it moves from East to West, and the blue front face reflection moves from West to East. When the prism is normal to the sun, the red and blue reflections coincide.
An added refinement is a cylindrical lens of 1.5m focal length. This has the property of giving the reflected bars of light sharp vertical edges. They do not look that sharp in the movie because the camara did not really have anything to focus on. But in real life the edges are quite sharp.
It might be better to time the moment when the two bars touch, rather than when the two bars are coincident, in the same way that sun moon sextant sightings are done.
I will post a photo of "the apparatus" tomorrow.
Dave Pike brought up the question of how to align the instrument. This, I suspect, is why the Dipleidoscope never caught on, as the paper by Boys relates. It is relatively easy to align if you already have an accurate clock with which to time the moment of local noon and so set the intrument. But doing so without a clock is much more of a challenge. There is a way in which it could be aligned to within a minute of arc using not much more than a candle and piece of string. To see how that works, I refer you to a paper by G. Kolbe, "A Test of the Simultaneous Transit Method", JHA, XXXIX, pp 515-517, (2008)
This URL will bring it up https://sci-hub.hkvisa.net/10.1177/002182860803900406