Appendix 6
The Rochon Micrometer Telescope
William E.Logan first used the Rochon micrometer telescope in Canada in 1844, to measure distances along streams and the coastline of the peninsula.
He used it again in1845, to measure distances along the Ottawa and Mattawa rivers.‡
The concepts behind the Rochon micrometer telescope developed over two centuries.
In1669 Erasmus Bartholinus discovered, that when a ray of light is refracted by a crystal of calcite it forms two refracted rays.
Historically, calcite was the point of departure for the development of a crystal micrometer. The problem of applying the birefringent properties of calcite to a double image micrometer remained with eighteenth and nineteenth century astronomers, until
M. de Rochon devised a method of applying the two images formed by double refraction to the construction of a micrometer. (Jour. de phys.,1801, pp. 169198)
The Rochon micrometer was developed and used by astronomers to measure the diameters of the planets, and by mariners to measure distances during naval operations. (1,2)‡ [Insert photo of Logan’s Rochon micrometer telescope]
The telescope contained two wedges of rock crystal ( ), cut at appropriate directions–
and cemented together to increase their double refraction .(3)
The rock crystal (prism) was located between the eyepiece and the object glass of the telescope and moved back and forth in the telescope by means of a slit along its length. A vernier scale along the slit allowed optical angles to be measured up to 40 minutes of a degree, to the nearest second.‡
The prism formed two images of a vertical object being sighted (a graduated pole, staff, or rod). The two images were brought into contact by adjusting the distance of the prism from the eyepiece, and the variation of the optical angle subtended by the object was indicated by the vernier scale located on the outside of the telescope, at the eyepiece end. Knowing the height of the object being sighted, the distance could be calculated by simple geometry, using a conversion table that converted the angle to distance from the viewer.‡
Logan purchased his micrometer telescopes from W. H. Jones, an optician, in London, England. Booklet on their use, with conversion tables, was printed by John Lovell in Montreal. Logan graphically describes his use of the micrometer telescope during the 1844 field season, with a small sketch of a man holding a vertical pole to which two oblong pieces of wood are nailed just five feet apart.‡0
I have with me a micrometer tellescope‘,by means of which a distance can be measured through the contrivance ...... (in other copy of this text ) [Insert full text here]
The Geological Survey ordered additional Rochon telescopes for its field parties. Micrometer surveys became an important method of topographical surveying in Canada, along with transit or compass and chain surveys, triangulation, paced surveys, and track or timed surveys.(5)‡
The evolution of the micrometer target rod, from that used by Logan in 1844, is seen in
pictorially demonstrated by the following photo of. P.Low’s geological party on Lake Mistassini in 1884. [Insert Low party photo here, with GSC credit]
...... …
References
(1) (Insert Brook Observatory reference here)
(Also 1911 encyclopedia‘_reference?).
(2) Jones, _(1842) On the Rochon Micrometer Telescope, and its Application to the Measurement of Distances; with a Table of the Distances for every Second. Published and Sold by Mr. Jones, Optician, 62 Charing Cross, London: Johnđ_Lovell St. Nicholas Street, Montreal.
(3) Wollaston, William Hyde (1820). On the methods of cutting rock crystals for micrometers: Philosophical Transactions of the Royal Society of London, v.110, pp. 126-131].
(4) Logan William E. (1844) Journal, May 21Sept. 10, 1844: National Library of Wales 21716B (or‘_Microfilm Reel A1658)‡
(5) White, James Tyrrell, J. B. (1897) The Topographical Work of the Geological Survey of Canada: Geog. J., London, December, pp.623-630.