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
.
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, 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:
| Number of the Star. | First Co-ordinate. | Second Co-ordinate. | ||
|---|---|---|---|---|
| Observed. | Calculated. | Observed. | Calculated. | |
| 11 | -0.19 | -0.22 | +0.16 | +0.02 |
| 5 | +0.29 | +0.31 | -0.46 | -0.43 |
| 4 | +0.11 | +0.10 | +0.83 | +0.74 |
| 3 | +0.20 | +0.12 | +1.00 | +0.87 |
| 6 | +0.10 | +0.04 | +0.57 | +0.40 |
| 10 | -0.08 | +0.09 | +0.35 | +0.32 |
| 2 | +0.95 | +0.85 | -0.27 | -0.09 |
(c) DISPLACEMENT OF SPECTRAL LINES TOWARDS THE RED
In XXIII it has been shown that in a system
' which is in rotation with regard to a Galileian system