It is shown here that the large-scale component of interstellar scintillation which causes the slow variation in the apparent intensity of pulsars on time scales of days to months can be greatly enhanced when the turbulence spectrum has a limiting inner scale of the order 10 to the 9th m. This solution is extended to the spherical diverging geometry appropriate for a pulsar embedded in the ISM, and a similar enhancement is found. A comparison of this solution with pulsar observations is used to place bounds on the inner scale of the ISM. 39 references.