It was first proposed in Traite de la Lumiere and is only valid for an odd number of spatial dimensions; however, the case of one spatial dimension is degenerate. Gustav Kirchoff demonstrated how it could be obtained from Maxwell's equations and the unresolved issue of backward propagation was expounded upon in Feynman-Wheeler theory. For an isotropic medium, it states that the three dimensional envelope of a point source can be either spherical or ellipsoidal, and to account for diffraction, it was modified to describe the amplitude at a point as the superposition of 
It was first proposed in Traite de la Lumiere and is only valid for an odd number of spatial dimensions; however, the case of one spatial dimension is degenerate. Gustav Kirchoff demonstrated how it could be obtained from Maxwell's equations and the unresolved issue of backward propagation was expounded upon in Feynman-Wheeler theory. For an isotropic medium, it states that the three dimensional envelope of a point source can be either spherical or ellipsoidal, and to account for diffraction, it was modified to describe the amplitude at a point as the superposition of all secondary amplitudes by Fresnel. FTP, give this theory which states that spherical wavelets emanating from points on a wavefront conform to the next wavefront of a propagating wave.
It was first proposed in Traite de la Lumiere and is only valid for an odd number of spatial dimensions; however, the case of one spatial dimension is degenerate.
Huygens' Principle [or Huygens-Fresnel Principle]