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 [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.
The results of the measurements confirmed the theory in a thoroughly
satisfactory manner. The rectangular components of the observed and of
the calculated deviations of the stars (in seconds of arc) are set
forth in the following table of results:
image054
(_c_) Displacement of Spectral Lines Towards the Red
In Section XXIII it has been shown that in a system _K _ which is in
rotation with regard to a Galileian system _K_, clocks of identical
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