
INTERFERENCE 191
of materials, and even being careful to minimize the effects of air currents with the attendant
changes in index of refraction. For other applications, careful mechanical design is required to
maintain alignment. It is important to note that the alignment constraints apply to the relative
alignment of the two paths, and not to the alignment of the source or observer with respect to
the interferometer. After carefully designing and building our interferometer on a sufciently
stable platform, we can, for example, carelessly balance the light source on a pile of books
and we will still observe stable fringes.
7.1.5 Balanced Mixing
Thus ...