We carried out observations of pulsar PSR B1919+21 at 324 MHz to study the distribution of interstellar plasma in the direction of this pulsar .
We used the RadioAstron ( RA ) space radiotelescope together with two ground telescopes : Westerbork ( WB ) and Green Bank ( GB ) .
The maximum baseline projection for the space-ground interferometer was about 60000 km .
We show that interstellar scintillation of this pulsar consists of two components : diffractive scintillations from inhomogeneities in a layer of turbulent plasma at a distance z _ { 1 } = 440 pc from the observer or homogeneously distributed scattering material to pulsar ; and weak scintillations from a screen located near the observer at z _ { 2 } = 0.14 \pm 0.05 pc .
Furthermore , in the direction to the pulsar we detected a prism that deflects radiation , leading to a shift of observed source position .
We show that the influence of the ionosphere can be ignored for the space-ground baseline .
Analysis of the spatial coherence function for the space-ground baseline ( RA-GB ) yielded the scattering angle in the observer plane : \theta _ { scat } = 0.7 mas .
An analysis of the time-frequency correlation function for weak scintillations yielded the angle of refraction in the direction to the pulsar : \theta _ { ref, 0 } = 110 ms and the distance to the prism z _ { prism } \leq 2 pc .