  This paper investigates the effects that perturbations to an optical system,
such as translations or rotations of the optical elements, have on the final
location where a light ray strikes a detector. Symmetry arguments are employed
to give selection rules for the combinations of those perturbations that can
affect the light ray. It is shown that a ``Transverse Parity'' can be assigned
to each type of perturbation and that only combinations with an overall odd
parity will produce an effect. Some examples of how these results can be
applied to undergraduate classroom or laboratory courses are given.
