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In a number of multiply imaged quasar systems, a significant contribution tothe lensing potential is provided by groups and clusters of galaxies associatedwith the primary lens. As part of an ongoing effort to gather observationaldata on these systems, we present spectroscopy and near-infrared and opticalphotometry of galaxies in the field of the quadruple lens system B 1422+231.The spectra show that the primary lens and five nearby galaxies belong to acompact group at z = 0.338. The median projected radius of this group is 35h^{-1} kpc and its velocity dispersion is 550 km/s. A straightforwardapplication of the virial theorem yields a group mass of 1.4 x 10^{13} h^{-1}M(sun), which provides sufficient external shear to produce the observed imageconfiguration. This data rules out a class of models and improves the system'sprospects for a measurement of the Hubble constant.Comment: 16 pages including 3 tables, 2 eps figures and 2 jpeg images.  Submitted to the Astronomical Journa

The External Shear Acting on Gravitational Lens B1422+231