Letter from James Gregory to John Collins, dated 13 May 1673. Reprinted in: Correspondence of Scientific Men of the Seventeenth Century...., ed. Stephen Jordan Rigaud (Oxford, England: Oxford University Press, 1841), vol. 2, pages 251–255; see especially page 254. Available on-line at: Books.Google.com.
Ceres Amson, John (May 2008). "Gregory's meridian line of 1673–74: a St Andrews detective story". BSHM Bulletin: Journal of the British Society for the History of Mathematics. 23 (2): 58–72. doi:10.1080/17498430802019804. S2CID218589286.
Turnbull 1939, pp. 168–174. Dehn, M.; Hellinger, E. (1943). "Certain Mathematical Achievements of James Gregory". American Mathematical Monthly. 50 (3): 149–163. doi:10.2307/2302394. JSTOR2302394. Turnbull, Herbert Westren, ed. (1939). James Gregory; Tercentenary Memorial Volume. G. Bell & Sons for the Royal Society of Edinburgh.
Turnbull 1939, pp. 168–174. Dehn, M.; Hellinger, E. (1943). "Certain Mathematical Achievements of James Gregory". American Mathematical Monthly. 50 (3): 149–163. doi:10.2307/2302394. JSTOR2302394. Turnbull, Herbert Westren, ed. (1939). James Gregory; Tercentenary Memorial Volume. G. Bell & Sons for the Royal Society of Edinburgh.
Ceres Amson, John (May 2008). "Gregory's meridian line of 1673–74: a St Andrews detective story". BSHM Bulletin: Journal of the British Society for the History of Mathematics. 23 (2): 58–72. doi:10.1080/17498430802019804. S2CID218589286.