  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 $z_{prism} \le 2$ pc.
