The damage threshold for three optical glasses was found to be higher for circularly polarized radiation than for linear polarization, using 26‐nsec pulses from a Nd:glass laser. The damage was assumed to result from self‐focusing. The fractional contribution of electrostriction to the total nonlinear indexn2is estimated to be 0.8 ± 0.2 for borosilicate crown glass, 1.15 ± 0.35 for fused silica, and 0.4 ± 0.1 for dense flint glass, assuming the Kerr effect to be the only other self‐focusing mechanism. The data are consistent with present concepts of glass structure. The high damage threshold in fused silica is attributed to its relatively small Kerr effect.