The invention features a system for microlithography that includes a mercury light source configured to emit radiation at multiple mercury emission lines, a projection objective positioned to receive radiation emitted by the mercury light source, and a stage configured to position a wafer relative to the projection objective. During operation, the projection objective directs radiation from the light source to the wafer, where the radiation at the wafer includes energy from more than one of the emission lines. Optical lens systems for use in said projection objective comprise four lens groups, each having two lenses comprising silica, the first and second lens groups on one hand and the third and fourth lens groups on the other hand are positioned symmetrically with respect to a plane perpendicular to the optical axis of said lens system.