The design pre-dates the first practical reflecting telescope, the Newtonian telescope, built by Isaac Newton in 1668, but was not successfully built until five years after Newton’s first reflecting telescope.
Gregory described his design in his 1663 book Optica Promota. The instrument employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image, useful for terrestrial observations.
In the same book Gregory also outlined how the principle and his telescope could be used to observe celestial events such as the transit of Venus (the most recent one having taken place in 1639, a year after Gregory’s birth), and could therefore help measure the distance of the Earth from the Sun.
These ideas were subsequently taken up and advocated by Edmond Halley in 1716, when he called on the global scientific community to make preparations for the 1761 Venus transit, knowing that he wouldn’t be alive to witness the event himself. Halley never credited Gregory for providing his inspiration, however.
The above top photo, is of a portable Gregorian telescope made by another Scottish astronomer, James Short, almost a century after Gregory’s death. It’s similar to one used by James Cook and the Endeavour’s astronomer Charles Green on their voyage to Tahiti to observe the 1769 transit of Venus.
The above right diagram, of ‘An Apparatus adapted to the Reflecting Telescope for shewing The Transit of Venus’, aimed to explain the principle employed to aspiring amateur astronomers.
It appeared in Benjamin Martin’s The Young Gentleman and Lady’s Philosophy, a fictional story published between 1755 and 1763 in the General Magazine.
Martin, like other popular scientists of the time such as James Ferguson, helped spread Enlightenment teachings through a pseudo-Socratic dialogue between his characters – college student Cleonicus, who educated his sister Euphrosine on astronomy, globes, optics and telescopes, and many other subjects.
In the mid-18th century, women couldn’t attend universities and so Euphrosine was very grateful to her brother. “How happy will be the age when the ladies may modestly pretend to knowledge,” she proclaimed in one extract, according to a past exhibition at the Adler Planetarium.
The photo of James Short’s portable Gregorian telescope comes from the Royal Museums Greenwich, while the diagram from The Young Gentleman comes via Wellcome Images. A digitized version of the entire publication, compiled in 1781 towards the end of Martin’s life, is available at Google Books.