The ornamental design for a viewing aid, as shown and described.
My viewing aid is characterized by an oval area containing a pattern comprising concentric circular contours. My viewing aid is further characterized by a rim of rectangular cross section abutting said oval area. My viewing aid is further characterized by a linear portion of cylindrical cross section separated from said oval area by a rectangular gap.
FIG. 1 is a front elevation view of a viewing aid showing my new design;
FIG. 2 is a left side elevation view of one edge thereof, the right side elevation view being a mirror image; and,
FIG. 3 is a side elevation view of a section along the line A-B of FIG. 1, showing the surface detail.