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which is a measure for the "distance" to the centre. As to and , we shall put , , after first having introduced polar coordinates (in such a way that the rectangular coordinates are , , ). It can be proved that, because of the symmetry about the centre, for , while we may put for the quantities


where are certain functions of . Ditferentiations of these functions will be represented by accents. We now find that of the complex only the components , and are different from zero. The expressions found for them may be further simplified by properly choosing . If the distance to the centre is measured by the time the light requires to be propagated from to the point in question, we have . One then finds


§ 49. We must assume that in the gravitation fields really existing the quantities have values differing very little from those which belong to a field without gravitation. In this latter we should have

and thus we put now

where the quantities and which depend on are infinitely small, say of the first order, and their derivatives too. Neglecting quantities of the second order we find from (81)

For our degree of approximation we may suppose that of the quantities only differs from 0. If we put