The ornamental design for a stereoscopicviewer, as shown and described.
FIG. 1 is a front perspective view of a stereoscopic viewer showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof; and,
FIG. 7 is a rear elevational view thereof.
Broken line illustrations of three corner flaps in FIGS. 3, 5 and 6 and various edge tabs in FIGS. 1-6 are not part of the design sought to be patented.