  The flux of the [OIII] line is considered to be a good indicator of the
bolometric emission of quasars. The observed continuum emission from the
accretion disc should instead be strongly dependent on the inclination angle
theta between the disc axis and the line of sight. Based on this, the
equivalent width (EW) of [OIII] should provide a direct measure of theta. Here
we analyze the distribution of EW([OIII]) in a sample of ~6,000 SDSS quasars,
and find that it can be accurately reproduced assuming a relatively small
intrinsic scatter and a random distribution of inclination angles. This result
has several implications: 1) it is a direct proof of the disc-like emission of
the optical continuum of quasars; 2) the value of EW([OIII]) can be used as a
proxy of the inclination, to correct the measured continuum emission and then
estimate the bolometric luminosity of quasars; 3) the presence of almost
edge-on discs among broad line quasars implies that the accretion disc is not
aligned with the circumnuclear absorber, and/or that the covering fraction of
the latter is rather small. Finally, we show that a similar analysis of EW
distributions of broad lines (Hbeta, Mg II, C IV) provides no evidence of
inclination effects, suggesting a disc-like geometry of the broad emission line
region.
