should really be written
, since
refers to a contravariant vector.
[92] There exist types of fields for which the potential distribution is indeterminate, but we need not consider this case, as Newton’s gravitational field is of the kind which admits a definite potential distribution.
[93] In four-dimensional space-time there are sixteen of these
’s at every point, but, the tensor being symmetrical, six turn out to be mere repetitions, so we need only speak of ten separate