FIG. 1 is a perspective view of a lamp embodying the design of the present invention. FIG. 2 is a front side elevational view of the lamp illustrated in FIG. 1. FIG. 3 is a right side elevational view of the lamp of FIG. 1, with the left side elevational view being a mirror image thereof. FIG. 4 is a back side elevational view of the lamp of FIG. 1. FIG. 5 is a top view of the lamp of FIG. 1; and, FIG. 6 is a bottom view of the lamp of FIG. 1. The lamp shade shown in broken lines does not form a part of the claimed design.