Lensing by Distant Clusters: [ITAL]HST[/ITAL] Observations of Weak Shear in the Field of 3C 324

We present the detection of weak gravitational lensing in the field of theradio galaxy 3C324 (z=1.206) using deep HST imaging. ~From an analysis of theshapes of faint R=24.5-27.5 galaxies in the field we measure a weak, coherentdistortion centered close to the radio source. This shear field most likelyarises from gravitational lensing of distant field galaxies by a foregroundmass concentration. In the light of previous observations of this region, whichindicate the presence of a rich cluster around the radio source, we suggestthat the most likely candidate for the lens is the cluster associated with theradio galaxy at z=1.2. If so, this is the most distant cluster to have beendetected by weak shear observations. Such a statement has two importantconsequences. Firstly, it shows that massive, collapsed structures exist in thehigh redshift Universe, and secondly that a significant fraction of theR=24.5-27.5 field galaxy population lies beyond z=1.2.Comment: 6 pages, uuencoded, compressed Postscript (no Figure 1), accepted for publication in ApJL. Postscript version including additional plate (329 Kb) available at ftp://ociw.edu/pub/irs/pub/3c324.ps.