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In addition, a second photograph of the same stars is taken when

the sun is situated at another position in the sky, i.e. a few

months earlier or later. As compared with the standard

photograph, the positions of the stars on the eclipse-photograph

ought to appear displaced radially outwards (away from the centre

of the sun) by an amount corresponding to the angle a.

We are indebted to the [British] Royal Society and to the Royal

Astronomical Society for the investigation of this important

deduction. Undaunted by the [first world] war and by difficulties

of both a material and a psychological nature aroused by the war,

these societies equipped two expeditions-to Sobral (Brazil), and

to the island of Principe (West Africa)-and sent several of

Britain's most celebrated astronomers (Eddington, Cottingham,

Crommelin, Davidson), in order to obtain photographs of the solar

eclipse of 29th May, 1919. The relative discrepancies to be

expected between the stellar photographs obtained during the

eclipse and the comparison photographs amounted to a few

hundredths of a millimetre only. Thus great accuracy was

necessary in making the adjustments required for the taking of

the photographs, and in their subsequent measurement.