The ornamental design for a speculum, as shown and described.
FIG. 1 is a front top perspective view of a speculum showing my new design;
FIG. 2 is a bottom rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof:
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The dot-dash broken lines represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.