The ornamental design for a speculum, as shown and described above.
FIG. 1 is a perspective view of a speculum, showing our new design;
FIG. 2 is a left side elevational view thereof, whereas the right side elevational view is a mirror image;
FIG. 3 is a top plan view thereof;
FIG. 4 is rear elevational view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a front elevational view thereof.
The broken lines shown in FIGS. 1, 3 and 4 indicate that the interior portions defined thereby are disclaimed. The broken lines show environmental structure only and form no part of the claimed design.