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A Royal Society Council meeting, held on 28 October 1897, recorded an outgoing loan:

One hundred and twenty-nine years later, two historic glass plates from 1875 have been found and returned to the Royal Society, coinciding with the .

Back in July, I was contacted by our archivist colleagues at the (RAS), who had rediscovered the slides in their collection and were offering to return them. In 1897, when the slides were originally loaned, the Royal Society was housed in the east wing of Burlington House, just across the courtyard from the RAS. The two societies have long worked closely together, sharing common interests and Fellows, making it rather likely that someone ‘popped over’ to drop the slides off, with the intention of returning them in a similar fashion. Over a century later (and with no overdue fines for the RAS), we’re grateful for their retrospective cataloguing efforts which led to the rediscovery.

The slides on a light box at the RAS, with the original loan slip

Delving further into the historical context of the slides, I learned that the Royal Society undertook to organise an expedition to observe the , encouraged by King Chulalongkorn (Rama V), the King of Siam [Thailand]. His father, King Mongkut (Rama IV), had been an enthusiastic astronomer, calculating and participating in solar eclipse measurements in 1868. As recorded in our Council Minutes ():

‘£300 from the Donation Fund to be used for the cost of instruments supplied to the Eclipse Expedition. Draft letter to the first Lord of the Treasury, noting the Admiralty's willingness to assist in an Eclipse Expedition, offering one of their ships in the Gulf of Siam, and noting the co-operation of the Government of India, and of the King of Siam: the Royal Society will provide instruments by loan and purchase, and several gentlemen have offered to go as unpaid volunteers; but a further £1,000 is needed and requested.’

Forwarded invitation from the King of Siam for members of the Royal Society to observe the eclipse in Thailand in April 1875 ()

Astronomer was to lead the expedition, but his duties as secretary of the Royal Commission on Scientific Instruction prevented him from going. The daunting task fell to (later FRS), only 23 years old at the time. He was officially appointed by the Royal Society on 4 February 1875, with the resolution that ‘the whole of the observations taken by the Siam party be considered the property of the Royal Society and sent to that body for discussion’.

The expedition was formed of two parties, one heading to Siam under Schuster and the other, under (later FRS), destined for India’s Nicobar Islands. Schuster was accompanied by Frank Edward Lott and Frederick Beasley, the latter an amateur photographer. Quite a small and inexperienced team for an important international expedition.

In his , published in 1932, Schuster candidly recounted his experience of the expedition, including a moment of intense dread just before the eclipse, when he realised that his whole future depended upon some slight moments in time, and a few fragile pieces of glass:

‘It was a cruel position for a young man to be placed in who knew, or at any rate felt convinced, that failure would mean ruin of his future career. I could not help sitting down and having a good cry … I almost hoped that the weather, which was a little threatening, would turn bad and relieve me of all responsibility’.

It was just trepidation in the end, as a small family of supporters came to Schuster’s aid; both local Thai officials and villagers, and expatriate Europeans. One was Captain Alfred John Loftus (1834-1899), then hydrographer to the King of Siam, who with his wife ‘took charge of the small camera, by means of which a set of photographs of the corona were obtained’. The exposure times were suggested by the French astronomer (1824-1907) and the resulting double set of four photographs is represented on our returned plates. These were developed by Francis Chit (1830-1891), thankfully a professional, and the court photographer to Rama V. With the German-born Arthur Schuster, it was a text-book piece of international scientific co-operation.

Magnified image on one of the 1875 plates

Schuster remarked that the corona was not what he expected based on the descriptions of others, observing that ‘instead of showing a soft silvery light, it struck me as being bright, almost dazzling ... like the false rays you see when you look towards the sun, and I could well understand some of the early observers thought the corona has no normal existence due to irregular refraction in our atmosphere’.

Arthur Schuster by William Orpen, 1912 ()

The observations were a success, and the prismatic camera captured eight good photographs of the corona, despite challenges presented by a hazy sky. Lockyer and Schuster produced an expedition report and in the Philosophical Transactions of the Royal Society, identifying that ‘the corona surrounded the moon like a regular and well-defined star’. Schuster received a personal letter from Phya Bhaskarawongse (1851–1920), Ambassador of the King of Siam, expressing how glad he was to hear of the success of the eclipse observations:

(L) the Ambassador’s letter, ; (R) ‘Spy’ cartoon of Phya Bhaskarawongse, Vanity Fair, September 1879 (Leslie Ward, public domain via )

Schuster’s observing days were not over. In 1878 he , and he would go on to have an illustrious career as a physicist. He became Secretary, then Foreign Secretary of the Royal Society, was an important figure in nurturing international scientific relations after the First World War. A stellar career, you might say.