314 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. C. DAVIDSON ON A 



unconnected with 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 

 being two or three degrees inclined to the hprizontal. A canvas screen was arranged 

 to protect the tube and object-glass from the direct radiation of the sun. 



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. Exposures 

 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 Principe 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 ccelostat) on a 

 terrestrial mark, and then starting the clock at a particular instant. 



