In 1919 two expeditions were dispatched from Britain to observe a total eclipse of the sun, the object being to test Einstein’s general law of relativity by determining what effect, if any, is produced when the path of a ray of light crosses a gravitational field. One party, comprising A. C. D. Crommelin and C. Davidson, went to Sobral, a town in the north of Brazil; the other, comprising Eddington and E. T. Cottingham, went to Principe, a small island off the west coast of Africa. The present group of letters, written by Eddington to his mother and sister, contains an account of his part in the latter expedition.
The four observers left Liverpool together aboard the steamship Anselm on 8 March and arrived at Madeira on the 12th, where they parted. Crommelin and Davidson went on to Brazil aboard the Anselm, while Eddington and Davidson were obliged to stay at Madeira till 9 April, when they recommenced their journey aboard the Portugal. They arrived at S. Antonio in Principe on the 23rd. After inspecting various possible sites on the island, they settled on Roça Sundy, the headquarters of a plantation owned by Senhor Carneiro, and their baggage was transported there on the 28th. They spent a week preparing the equipment, before returning to S. Antonio for the week 6–13 May; they then went back to Sundy to continue their preparations. The eclipse took place on 29 May. On 12 June the observers left Principe on the steamship Zaire. After changing ships at Lisbon, they arrived at Liverpool on 14 July. A report of the expeditions was communicated to the Royal Society on 30 October and printed the following year (Phil. Trans. A, ccxx (1920), 291–333). A draft by Eddington of the part of the report relating to the Principe expedition will be found at C1/3.