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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 (Philosophical Transactions A, vol. ccxx (1920), pp. 291–333). A draft by Eddington of the part of the report relating to the Principe expedition will be found at C1/3.

In 1912 Eddington led an expedition from the Royal Observatory to Brazil to observe and photograph a total eclipse of the sun. These letters contain an account of the expedition. Eddington specifically asked his mother to preserve the letters because he was keeping no other record of events (see A2/5), and she seems to have been careful in carrying out his request, as the sequence is probably complete. The letters are numbered from 1 to 9 in a contemporary hand, and the ninth letter, finished when the writer was about three days’ sail from home, was almost certainly the last. These letters formed the basis of the account in Eddington’s Notebook (Add. MS b. 48, ff. 96–102), and they were consulted by Douglas, who quoted from A2/8 (pp. 18–19).

Eddington and his assistants C. R. Davidson and J. J. Atkinson left Southampton aboard the steamship Arlanza on 30 August and arrived at Rio de Janeiro on 16 September, where they were joined by T. N. Lee, an Englishman deputed by the Brazilian Government to assist them, and J. H. Worthington, an amateur astronomer. Six days later the party arrived at Passa Quatro, their chosen observation point—preferred to other possible sites at Cruxeiro, Christina, and Alfenas—and on 3 October they were joined by two volunteers, Leslie Andrews and O. Couto de Aguirre. A local man, Pierre Seux, also volunteered to help by counting seconds during totality. The eclipse took place on the 10th, but unfortunately observations of the phenomenon were prevented by rain and the expedition was largely unsuccessful. Eddington and his companions left Passa Quatro on 20 October and sailed for home on the Danube on the 23rd. The date of their arrival in England is not recorded, but towards the end of the voyage they were expecting to be at Southampton on 10 November. A report of the expedition, by Eddington and Davidson, was printed in MNRAS (lxxiii, 386–90) the following year. Notes also appeared in The Observatory, xxxv (1912), 328–30, 410, and xxxvi (1913), 62–5.