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Since it has often been contended by opponents of the theory of

relativity that the special theory of relativity is overthrown by

the general theory of relativity, it is perhaps advisable to make

the facts of the case clearer by means of an appropriate

comparison. Before the development of electrodynamics the laws of

electrostatics were looked upon as the laws of electricity. At

the present time we know that electric fields can be derived

correctly from electrostatic considerations only for the case,

which is never strictly realised, in which the electrical masses

are quite at rest relatively to each other, and to the

co-ordinate system. Should we be justified in saying that for

this reason electrostatics is overthrown by the field-equations

of Maxwell in electrodynamics? Not in the least. Electrostatics

is contained in electrodynamics as a limiting case; the laws of

the latter lead directly to those of the former for the case in

which the fields are invariable with regard to time.