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If we confine the application of the theory to the case where the

gravitational fields can be regarded as being weak, and in which

all masses move with respect to the coordinate system with

velocities which are small compared with the velocity of light,

we then obtain as a first approximation the Newtonian theory.

Thus the latter theory is obtained here without any particular

assumption, whereas Newton had to introduce the hypothesis that

the force of attraction between mutually attracting material

points is inversely proportional to the square of the distance

between them. If we increase the accuracy of the calculation,

deviations from the theory of Newton make their appearance,

practically all of which must nevertheless escape the test of

observation owing to their smallness.