almost an exact circle, which makes it more difficult to locate the perihelion with precision.

In point of fact, astronomers have found that the theory of Newton does not suffice to calculate the observed motion of Mercury with an exactness corresponding to that of the delicacy of observation attainable at the present time. After taking account of all the disturbing influences exerted on Mercury by the remaining planets, it was found (Leverrier–-1859–-and

Newcomb–-1895) that an unexplained perihelial

movement of the orbit of Mercury remained over, the amount of which does not differ sensibly from the above-mentioned $+ 43$ seconds of arc per century. The uncertainty of the empirical result amounts to a few seconds only.

Deflection of Light by a Gravitational Field

In [chapter:XXII]Section XXII it has been already mentioned that,

according to the general theory of relativity, a ray of light will experience a curvature of its path when passing

through a gravitational field, this curvature being similar to that experienced by the path of a body which is projected through a gravitational field. As a result of this theory, we should expect that a ray of light which is passing close to a heavenly body would be deviated towards the latter. For a ray of light which passes the sun at a distance of $Δ$ sun-radii from its centre, the angle of deflection ( $α$ ) should amount to $α = {$ 1.7 $s e c o n d s o f a r c}} Δ .$ It may be added that, according to the theory, half of this deflection is produced by the Newtonian field of attraction of the sun, and the other half by the geometrical modification ("curvature") of space caused by the sun.

1in127 This result admits of an experimental

test by means of the photographic registration of stars during a total eclipse of the sun. The only reason why we must wait for a total eclipse is because at every other time the atmosphere is so strongly illuminated by the light from the sun that the stars situated near the sun's disc are invisible. The predicted effect can be seen clearly from the accompanying 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 $α$ .

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