 As seen in Fig . 2a and 2b , the observer has a direct view of mirror M1 seen through the beam splitter , and sees a reflected image M ' 2 of mirror M2 . The fringes can be interpreted as the result of interference between light coming from the two virtual images S ' 1 and S ' 2 of the original source S. The characteristics of the interference pattern depend on the nature of the light source and the precise orientation of the mirrors and beam splitter . In Fig . 2a , the optical elements are oriented so that S ' 1 and S ' 2 are in line with the observer , and the resulting interference pattern consists of circles centered on the normal to M1 and M ' 2 . If , as in Fig . 2b , M1 and M ' 2 are tilted with respect to each other , the interference fringes will generally take the shape of conic sections ( hyperbolas ) , but if M1 and M ' 2 overlap , the fringes near the axis will be straight , parallel , and equally spaced . If S is an extended source rather than a point source as illustrated , the fringes of Fig . 2a must be observed with a telescope set at infinity , while the fringes of Fig . 2b will be localized on the mirrors . 
