Remarks upon the choice of co-ordinates.—It has already been remarked in §8, with reference to the equation (18a), that the co-ordinates can with advantage be so chosen that [sqrt](-g) = 1. A glance at the equations got in the last two paragraphs shows that, through such a choice, the law of formation of the tensors suffers a significant simplification. It is specially true for the tensor B_{μυ} should be ν? throughout], which plays a fundamental rôle in the theory. By this simplification, S_{μυ} vanishes of itself so that tensor B_{μυ} reduces to R_{μυ}.
I shall give in the following pages all relations in the simplified form, with the above-named specialisation of the co-ordinates. It is then very easy to go back to the general covariant equations, if it appears desirable in any special case.
C. THE THEORY OF THE GRAVITATION-FIELD
§13. Equation of motion of a material point in a gravitation-field. Expression for the field-components of gravitation.
A freely moving body not acted on by external forces
moves, according to the special relativity theory, along a
straight line and uniformly. This also holds for the
generalised relativity theory for any part of the four-dimensional
region, in which the co-ordinates K_{0} can be, and
are, so chosen that g_{μυ}'s have special constant values of
the expression (4).
Let us discuss this motion from the stand-point of any arbitrary co-ordinate-system K_{1}; it moves with reference to K_{1} (as explained in §2) in a gravitational field. The laws
