  Image flux ratio anomalies have been attributed to substructures within the
gravitational lens and to small mass halos (M <~ 10^10 Msun) in intergalactic
space. In this paper, analytic calculations are presented that help in the
understanding of how intergalactic halos affect magnification ratios. It is
found that intergalactic halos can produce anomalies at a similar level to
those that are observed. Intergalactic halos with masses <10^10 Msun are
expected to cause relative deflections between images of order 10
milliarcseconds, which are then magnified by the primary lens. They will also
cause fluctuations in the surface density on the several percent level. The
importance of intergalactic halos depends strongly on the radial profile of the
halos and the primordial power spectrum at small scales. Strongly lensed
quasars provide an opportunity to probe these properties. A strong dependence
on the QSO redshift is predicted and can be used to distinguish between
intergalactic structure and substructure as the cause of magnification
anomalies. This analytic approach also explains why some previous semi-analytic
estimates disagreed with numerical calculations.
