Pagina 155 van 212

The next step is to investigate the space-time behaviour of the

gravitational field G, which was derived from the Galileian

special case simply by transformation of the coordinates. This

behaviour is formulated in a law, which is always valid, no

matter how the reference-body (mollusc) used in the description

may be chosen.

This law is not yet the general law of the gravitational field,

since the gravitational field under consideration is of a special

kind. In order to find out the general law-of-field of

gravitation we still require to obtain a generalisation of the

law as found above. This can be obtained without caprice,

however, by taking into consideration the following demands:

(a) The required generalisation must likewise satisfy the general

postulate of relativity.

(b) If there is any matter in the domain under consideration,

only its inertial mass, and thus according to Section XV only its

energy is of importance for its effect in exciting a field.