  Montonen and Olive's conjecture in 1977 that some theories possess a duality
symmetry that interchanges the electrically charged particle with that of
negatively charged t' Hooft-Polyakov monopoles which relates strong coupling to
weak coupling. This will be nearly the same as Shapere et al, who showed that
the equation of motion of the coupled Einstein-Maxwell-axion dilaton system,
occurs in low energy in String theory, is invariant under electromagnetic
duality transformation; that also interchanges the strong and weak coupling
limit of the theory. These ideas have been persued in the $SO(6)\otimes SO(5)$
gauge groups as well as in the supersymmetric group of a four dimensional
String. We analyse and find that they give clearer evidence of string theory in
two and four dimensional supergravity theory. However, the application to
supergravity is beset with the normal difficulties and suggestions have been
made how to settle them.
