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Just google on iceland spar micrometer and you'll see a reference to Brayebrook Observatory.... worth reading. I have construction details at home, won't get to them until early Feb. So read up in the meantime. I can supply ( free!) a sample of iceland spar to get you started.
Brief summary:
The crystal produces a double image of the double star(!!??) the images are produced a fixed distance apart (depending on the thickness of the crystal, say 3 to 5 sec arc) so you can "estimate" distance by comparison. By rotating the eyepiece you can line up the two sets of images, this then gives the position angele.!!
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