  We have detected a strong deflection of radio waves from the pulsar PSR
B0834+06 in scintillation observations. Interference between the undeflected
pulsar image and deflected subimages allows single dish interferometry of the
interstellar medium with sub-milliarcsecond resolution. We infer the presence
of scattering structure(s) similar to those that are thought to cause Extreme
Scattering Events in quasar flux monitoring programs: size ~ 0.2 AU (an angular
size of 0.1 mas) with an electron density of > 10^3 compared to the warm
ionized medium. The deflectors are nearly stationary in a scattering screen
that is thin (< 5% of the pulsar-observer distance in extent), is located 70%
of the way from the Earth to the pulsar, and has been seen consistently in
observations dating back 20 years. The pulsar scans the scattering screen at a
velocity of 110 km/s with a detection radius of 15 mas. Pulsar observations
such as these -- particularly with a new generation of low-frequency radio
telescopes with large collecting areas -- hold promise for improving
constraints on the poorly understood physical characteristics and space density
of the deflecting structures. Such observations may also prove useful in
correcting deviations the deflectors produce in high-precision timing of
millarcsecond pulsars.
