  The Finnish physicist Gunnar Nordstr\"om developed a competing theory of
gravitation to Einstein's 1912-1913 gravitation theory. The equivalence
principle was valid in his theory and it also satisfied red shift of the
spectral lines from the sun. However, it was unable to supply the Perihelion of
Mercury, such as Einstein's theory; it led to a Perihelion like the one
predicted by Newton's law, and, it could not explain the deflection of light
near the sun, because in Nordstr\"om's theory the velocity of light was
constant. Einstein's 1913-1914 theory, the field equations of which were not
generally covariant, remained without empirical support. Thus a decision in
favor of one or the other theory - Einstein's or Nordstr\"om's - was impossible
on empirical grounds. Einstein began to study Nordstr\"om's theory from the
theoretical point of view and he developed his own Einstein-Nordstr\"om theory
on the basis of his conception of the natural interval. Eventually, in a joint
1914 paper with Lorentz's student Adrian Fokker, Einstein showed that a
generally covariant formalism is presented from which Nordstr\"om's theory
follows if a single assumption is made that it is possible to choose preferred
systems of reference in such a way that the velocity of light is constant; and
this was done after Einstein had failed to develop a generally covariant
formulation for his own theory.
