590 SCIENCE PROGRESS 



too far advanced to make it possible to alter the plans which 

 had been decided upon, even had it been possible to land the 

 astrographic telescope there. In Australia, farther to the 

 south-east, the sun was too low at the time of eclipse for any 

 station there to be considered. Christmas Island therefore 

 seemed the most suitable station. Although its climate was 

 not so favourable as that of the north-west coast of Australia, 

 where rain or cloud in September is almost unknown, it 

 appeared sufficiently favourable for the expedition to have a 

 good chance of success. September and October are usually 

 the two best months of the year with, as a rule, fairly settled 

 and dry weather. It was therefore selected as the station. 

 It was somewhat to the north of the central line of totality, 

 but by going as far south as possible on the island a duration 

 of nearly five minutes was obtainable, which was adequate, 

 given good weather, for an accurate determination of the 

 gravitational deflection. 



A new mounting for the astrographic telescope was obtained 

 from Sir Howard Grubb & Sons, suitable for the latitude of 

 the Island, to which the existing clockwork and electric con- 

 trol gear of the telescope was fitted. A new counterpoise 

 system was also provided. Special aluminium plate-holders 

 were designed and made at the Royal Observatory which could 

 be readily changed during the eclipse, and which enabled each 

 plate to take up a definite three-point bearing on the breech- 

 end of the telescope. 



The comparison plates of the eclipse field, which were to 

 be used in conjunction with the eclipse plates themselves for 

 the determination of the deflection, were to be obtained about 

 three months before the eclipse, when the field could be photo- 

 graphed shortly after sunset. It was decided to use the 

 opportunity of a large telescope mounted in a stable manner 

 near the Equator to combine with the eclipse some useful 

 astronomical work. This consisted of the determination of 

 the photographic magnitudes of stars in selected regions of 

 the southern sky, by taking photographs in which a southern 

 region was compared with a northern region. The magnitudes 

 of the stars in the northern regions which were to be used had 

 previously been determined at Greenwich by direct com- 

 parison with the standard area around the North Pole. A 

 means would thus be provided by which observers in the 

 southern hemisphere would be enabled to determine the 

 photographic magnitudes of stars on the scale which has been 

 standardised for the northern sky. Such means is at present 

 lacking. This programme required at least five months for 

 a sufficiently wide area of the southern sky to be covered, 

 which meant that, allowing about five or six weeks for the 



