314 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. C. DAVIDSON ON A 
unconnected witli the instrument. The telescope pointed slightly downwards, and the 
tube was turned so as to give the right orientation to the plate, the lines of declination 
])eing two or three degrees inclined to the horizontal. A canvas screen was arranged 
to protect the tube and object-glass from the direct radiation of the smi. 
The adjustments call for little comment. In view of the purpose of the observations, 
it was desirable to adjust the tilt of the object-glass and plate with special care. It was 
also important that the setting on the field should be nearly exact. The sun appeared 
on the eclipse day in sufficient time to allow of the setting being made by means of 
the solar image ; but arrangements had been tested by which the correct field would 
have been obtained if it had been cloudy up to totality.* The telescope was focussed 
by trial photographs of stars, and owing to the uniform temperature of the island the 
focus was unchanged for day observations. 
The object-glass was stopped down to 8 inches for the eclipse photographs and for 
all check and comparison photographs used in the reductions. 
29. The days preceding the eclipse were very cloudy. On the morning of May 29 
there was a very heavy thunderstorm from about 10 a.m. to 11.30 a.m.^—a remarkable 
occurrence at that time of year. The sun then appeared for a few minutes, but the 
clouds gathered again. About half-an-hour before totality the crescent sun was glimpsed 
occasionally, and by 1.55 it could be seen continuously through drifting cloud. The 
calculated time of totality was from 2h. 13m. 5s. to 2h. 18m. 7s. G.M.T. Exposimes 
were made according to the prepared programme, and 16 plates were obtained. 
Mr. CoTTiNGHAM gave the exposures and attended to the driving mechanism, and 
Prof. Eddington changed the dark slides. It appears from the results that the cloud 
must have thinned considerably during the last third of totality, and some star images 
were shown on the later plates. The cloudier plates give very fine photographs of a 
remarkable prominence which was on the limb of the sun. 
A few minutes after totality the sun was in a perfectly clear sky, but the clearance 
did not last long. It seems likely that the break-up of the clouds was due to the eclipse 
itself, as it was noticed that the sky usually cleared at sunset. 
It had been intended to complete all the measurements of the photographs on the 
spot ; but owing to a strike of the steamship company it was necessary to return by the 
first boat, if we were not to be marooned on the island for several months.- By the 
intervention of the Administrator berths, commandeered by the Portuguese Government, 
were secured for us on the crowded steamer. We left Principle on June 12, and after 
transhipping at Lisbon, reached Liverpool on July 14, 
30. The following is a list of the photographs, including the comparison photographs 
kindly taken for us by Mr. F. A. Bellamy at Oxford, before the instrument was 
dismounted. All the eclipse photographs are given, though only W and X furnished 
* The method depended on setting the cross-wires of the theodolite (attached to the coelostat) on a 
terrestrial mark, and then starting the clock at a particular instant. 
