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An optical imaging gas proportional counter with a capillary plate has been tested using a gas mixture of Ar+8%CH/sub 4/+2%TMA.. Both charge and light signals were investigated. The gas gain was found to be approximately 70 times greater than that previously obtained in Ar+10%CH/sub 4/ and the new mixture operates at a gap voltage 220 V lower than the Ar+10%CH/sub 4/, indicating the Penning effect of the trimethylamine (TMA) additive. The energy resolutions obtained for the charge signal and the light signal were 20.8% and 26.6% for /sup 55/Fe 5.9-keV X-rays, respectively. Through the relationship between gain and energy resolution for the light signal it was estimated that the light production rate was 1.2 photons per electron. Finally, from a cluster analysis of the photoelectron tracks due to 22-keV X-rays, it was inferred that the capillary gas proportional counter (CGPC) operating with Ar+8%CH/sub 4/+2%TMA could detect the starting point of the track, a necessary requirement for its eventual use as an imaging polarimeter.