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 diagram. If the sun ($$S$$) were not present, a star which is practically infinitely distant would be seen in the direction $$D_1$$, as observed from the earth. But as a consequence of the deflection of light from the star by the sun, the star will be seen in the direction $$D_2$$, i.e. at a somewhat greater distance from the centre of the sun than corresponds to its real position.

In practice, the question is tested in the following way. The stars in the neighbourhood of the sun are photographed during a solar eclipse.

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 Royal Society and to the Royal Astronomical Society for the investigation of this important deduction. Undaunted by the 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,, Crommelin, ), in order to obtain