We place limits on the linear power spectrum on small scales ( k \gtrsim 50 h \hbox { Mpc } ^ { -1 } ) using measurements of substructure in gravitational lens galaxies .
We find excellent agreement with the simplest \Lambda CDM models , and in conjunction with other cosmological probes , place constraints on the neutrino mass m _ { \nu } , tilt of the primordial power spectrum n , and mass of the dark matter particle m .
We find n > 0.94 , and for a Harrison-Zeldovich spectrum , m _ { \nu } < 0.74 eV and m > 5.2 keV , at 95 % confidence .