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Intra-Cluster Machos (ICMs) are a plausible candidate for at least some ofthe dark matter in clusters of galaxies. ICMs can be detected by searchingtoward M87 for ``pixel lensing'', gravitational microlensing of unresolvedstars. Dedicated observations by the Wide Field and Planetary Camera on the{\it Hubble Space Telescope} would discover lenses at a rate $\Gamma\sim 7f\,\rm day^{-1}$ where $f$ is the fraction of the Virgo cluster composed ofMachos (Massive Compact Objects). The only important background is lensing bystars in M87 itself. Such self-lensing should be detected at a rate $\Gamma\sim1\,\rm day^{-1}$. However, most of these background events should berecognizable from their angular distribution and/or measurement of their propermotions (which are smaller by a factor $\sim 2$ than those of ICMs). Theobservations should be carried out principally in $I$, but with $V$measurements made $\sim 20\%$ of the time in order to determine the colors ofthe lensed stars. It is also possible to detect ``Class II'' lensing eventswhich are longer and hence for which lower signal to noise is balanced by amuch larger number of observations per event. There are $\sim 5$ times more ofthese than the higher quality events. However, it is difficult to distinguishClass II events generated by ICMs from those generated by M87 stars.Comment: 16 pages, no figures, uuencoded postscript. Changes are mostly  related to including a large group of newly-recognized `Class II' events

Search for Intracluster MACHOs by Pixel Lensing of M87