through the distorted medium which would be straight if the dis- 
tortion were removed. Einstein has found that on this theory a 
modification is necessary in Newton’s law of gravitation. One 
form of Newton’s law is that expressed in Laplace’s potential 
function, but this form cannot be applied to such a force as centri- 
fugal force. To get uniformity of treatment of all varieties of 
force a modification is needed of the gravitational law from the 
statement as originally given by Newton. Einstein lias put for- 
ward a modified law which is indistinguishable from Newton’s law 
in its effects in all but a few crucial tests: that is to say, the 
modification has not upset in the slightest any of our customary 
deductions from the old form of the law of gravitation. On the 
other hand, using the old form of the law the motion of that point 
of the planet Mercury’s elliptical orbit round the Sun which lies 
nearest to the Sun was calculated to undergo a movement of 8 
minutes 52 seconds of arc per century. Observation, however, 
showed the movement to be at the rate of 9 minutes 34 seconds, 
and Einstein’s modification of the gravitational law has altered 
the calculated value to 9 minutes 35 seconds. Briefly put, Ein- 
stein’s theory has not upset one of the innumerable cases where 
the old law was in agreement with fact; it lias brought agreement 
in one ease (that of Mercury) where grave discrepancy existed, 
and in at least one case it has brought closer agreement than 
could previously he obtained. 
The forthcoming solar eclipse of 29th May, 1919, will afford 
an occasion for further testing Kinslein's theory, The Sun during 
totality will he in the constellation Taurus, and if photographed 
will he obtained surrounded hv certain stars to the north of the 
Hyades group. Now on Einstein’s theory light has not only inertia 
but has weight, that is to say, is subject to gravitational attraction. 
Accordingly, rays of light coming from stars A and / > (see dia- 
gram) will he deviated at I y and (/ so that they appear to come 
from stars situated at A/ and />'. These stars will therefore appear 
to he not merely a solar diameter PQ apart, hut at a rather greater 
separation P\)', and stars at B and C, which would otherwise be 
occulted by the intervening Sun will he visible at the Sun’s limb. 
On Einstein’s theory a distortion of 175 secs, of arc would he ex- 
pected from stars such as 4 or I). If no distortion is recorded 
we shall have the strange result of light possessing mass hut not 
weight, while a distortion of say 0*8 seconds would upset Ein- 
stein’s theory hut would show that light was subject to gravity 
Davidson and Cortie will observe the eclipse from Sobral in Brazil, 
while Cottingham and Eddington will be stationed on Principe 
Island off West Africa. The probable meteorological conditions 
are not too favourable, and at the best, some time will elapse before 
the photographic plates have been fully measured and compared 
with others of the same stars taken when the sun lias moved away 
