DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN'S GRAVITATIONAL FIELD. 295 



by Prof. BAUER, who was there on an expedition to observe magnetic effects. At the 

 station at Tanganyika it was thought the sun was at too low an altitude for observations 

 of this character, owing to the large displacements which would be caused by refraction. 

 A circular received from Dr. MORIZE, the director of the Observatory at Rio, stated 

 that Sobral was the most suitable station in North Brazil and gave copious information 

 of the meteorological conditions, mode of access, &c. 



7. Acting on this information the Joint Permanent Eclipse Committee at a meeting 

 on November 10, 1917, decided, if possible, to send expeditions to Sobral in North 

 Brazil, and to the island of Principe. Application was made to the Government Grant 

 Committee for 100 for instruments and 1,000 for the expedition, and a sub-committee 

 consisting of Sir F. W. DYSON, Prof. EDDINGTON, Prof. FOWLER and Prof. TURNER 

 was appointed to make arrangements for the expeditions. This sub-committee met in 

 May and June, 1918, and made provisional arrangements for Prof. EDDINGTON and 

 Mr. COTTINGHAM to take the object glass of the Oxford astrographic telescope to 

 Principe, and Mr. DAVIDSON and Father CORTIE to take the object glass of the Greenwich 

 astrographic telescope to Sobral. It was arranged for the clocks and mechanism of the 

 crelostats to be overhauled by Mr. COTTINGHAM. Preliminary inquiries were also set 

 on foot as to shipping facilities, from which it appeared very doubtful whether the 

 expeditions could be carried through. 



Conditions had changed materially in November, 1918, and at a meeting of the sub- 

 committee on November 8, it was arranged to assemble the instruments at Greenwich, 

 and make necessary arrangements with all speed for the observers to leave England by 

 the end of February, 1919. In addition to the astrographic object glasses fed by 16-inch 

 ccelostats, Father CORTIE suggested to the sub-committee the use of the 4-inch telescope 

 of 19-feet focus, which he had used at Hernosand, Sweden, in 1914, in conjunction with 

 an 8-inch coelostat, the property of the Royal Irish Academy. It was arranged to ask 

 for the loan of these instruments. As Father CORTIE found it impossible to spare the 

 necessary time for the expedition his place was taken by Dr. CROMMELIN of the Royal 

 Observatory. 



8. In November, 1918, the only workman available at the Royal Observatory was the 

 mechanic, the carpenter not having been released from military service. In these 

 circumstances Mr. BOWEN, the civil engineer at the Royal Naval College, was consulted. 

 He kindly undertook the construction of frame huts covered with canvas, which could 

 be easily packed and readily put together. These were generally similar to those used 

 in previous expeditions from the Royal Observatory (see ' Monthly Notices,' Vol. LVIL, 

 p. 101). He also lent the services of a joiner who worked at the Observatory on the 

 woodwork of the instruments. 



It was found possible to obtain steel tubes for the astrographic objectives. These 

 were, for convenience of carriage, made in two sections which could be bolted together. 

 The tubes were provided with flanges at each end, the objective being attached to one 

 of these, and a wooden breech piece to the other. In the breech piece suitable provision 



