Bifilar micrometer

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an instrument form measuring minute distances or angles by means of two very minute threads (usually spider lines), one of which, at least, is movable; - more commonly called a filar micrometer.

See also: Bifilar

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in periodicals archive ?
The traditional method is to use a bifilar micrometer, which has two very fine screws to move spider's webs across the field of view of the eyepiece by known amounts.
He centers Proxima in the hairs of an antique bifilar micrometer so viewers can tell which faint star it is.