As per the wave theory of light, the circular fringes appearing in the Michelson's Interferometer experiment are due to the interference of wavelets emanating from the two silvered mirrors in the apparatus. 

However these fringes are due to surface pattern on the silvered mirrors.

The Surface pattern Captured on a circular plain glass plate. The Surface pattern of the plain circular glass plate is shown in Fig-6a. 

 

 

Fig-6a.