Discovery of hidden blazars

Every radio-loud quasar may have blazar activities, according to a unified scheme where the differences in both optical and radio observations of radi o-loud quasars are the result of different viewing angles, We have predicte d that blazars may be detected using emission line ratio variations caused by variable illumination of gas clouds in the broad emission line region. I n a spectroscopic search of 62 quasars at a redshift of about 2, we have di scovered large (>20%) variations of the emission tine ratios, CIV/CIII] or CIV/Ly alpha, when compared with data taken more than 10 years ago. This re sult is consistent with our prediction and supports the unification scheme for radio-loud quasars.