The ornamental design for viewing device, as shown and described.
FIG. 1 is a front perspective view of a viewing device showing my new design, the undisclosed portions are not part of the claimed design;
FIG. 2 is a front view thereof;
FIG. 3 is a front view of a second embodiment of a viewing device showing my new design, the undisclosed portions are not part of the claimed design;
FIG. 4 is a front view of a third embodiment of a viewing device showing my new design, the undisclosed portions are not part of the claimed design; and,
FIG. 5 is a front view of a fourth embodiment of a viewing device showing my new design, the undisclosed portions are not part of the claimed design.
The portions of the viewing device shown in broken lines are for illustrative purposes only and form no part of the claimed design.