  Since dark matter clusters and groups may have substructure, we have examined
the sextupole content of Hubble images looking for a curvature signature in
background galaxies that would arise from galaxy-galaxy lensing. We describe
techniques for extracting and analyzing sextupole and higher weakly lensed
moments. Indications of substructure, via spatial clumping of curved background
galaxies, were observed in the image of CL0024 and then surprisingly in both
Hubble deep fields. We estimate the dark cluster masses in the deep field.
Alternatives to a lensing hypothesis appear improbable, but better statistics
will be required to exclude them conclusively. Observation of sextupole moments
would then provide a means to measure dark matter structure on smaller length
scales than heretofore.
