Eyepiece double-image spherical crystal micrometer using a made by George Dollond in 1830 for the Smyth telescope (1929-949 Pt1 ).

Captain W.H.Smyth used this Crystal micrometer to measure double stars for his famous book, 'A Cycle of Celestial Object' first published in 1844. The novel design of the instrument caused ambiguity in his readings which after his death resulted in accusation of inaccuracy and plagiarism. Charges that were shown subsequently to be unfounded (see 'An Astronomical Scrapbook' by Joseph Ashbrook, Cambridge 1984).