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    Re: Peculiar box sextant
    From: Luc Van den Borre
    Date: 2022 Sep 20, 15:02 -0700

    Joe Wong wrote:
    Came across this rather interesting piece on an online auction lot, and am trying to figure out the purpose of this instrument.

    It's a refractometer. The small cylinder contains two prisms. You open it up and put a drop of e.g. grape juice (if you're making wine) between them. The higher the concentration of sugar in the water, the more a light beam going through it bends.

    The mirror is for directing light from a sodium lamp through the sample. Sodium light has a single frequency so gives a nice sharp readout (refraction also depends on wavelength). There's a precise scale engraved on the glass under the eyepiece, invisible in the picture. The four rounded stubs on the small cylinder are attachments for rubber tubing to supply water, to bring the sample and the prisms to a required temperature (since refraction also depends on temperature).

    I'm not sure how the 0-180 adjustment on top of the alidade works. I guess it's used when calibrating the instrument for work with different solutions. "It has a revolving triple-prism compensator, like that in the Dipping refractometer, which is adjusted by a lever on the alidade.", says the first website I linked to. (Does that 0-180 scale span 36 degrees? Nice 5 to 1 ratio.)

    Cool but you can't navigate with it so meh.



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