Average HST-WFPC2 surface brightness profiles in the F851W filter (which measures the red stellar continuum) and the F658N filter (which includes the Hα+[N II] feature as well as some continuum) measured across the northern filament from NE to SW. We average the surface brightness profiles in a box of length 1″ which extends along the filament, centered in the location of our COS aperture. The filament is clearly visible above the red stellar continuum from M87. The relatively well-defined edges of the filament show that it has a roughly cylindrical geometry, as suggested by the imaging in Fig. 1, and it would require an unlikely geometrical coincidence for the filament to be a sheet or pancake seen in projection.