We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy .
Optical identifications have been obtained by matching together objects from the APM and FIRST surveys over the region 148.13 ^ { \circ } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2.0 pt% \hbox { $ < $ } } { RA ( 2000 ) } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } % \raise 2.0 pt \hbox { $ < $ } } 218.13 ^ { \circ } , -2.77 ^ { \circ } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2.0 pt% \hbox { $ < $ } } { Dec } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2 % .0 pt \hbox { $ < $ } } 2.25 ^ { \circ } .
Selecting radio sources down to 1 mJy , and adopting a uniform optical limit of b _ { J } = 21.5 we find 3176 have a counterpart in the APM catalogue , corresponding to 13 per cent of the radio sample .

For b _ { J } \leq 20.5 we can divide radio sources into resolved radio galaxies and stellar-like objects ( principally QSO ) .
We find the population of radio galaxies to be mainly made by early-type galaxies with very red colours ( b _ { J } - R up to \sim 4 ) and a radio-to-optical ratio 10 ^ { 2 } \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2.0 pt \hbox { $ < $ } % } r \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2.0 pt \hbox { $ < $ } } 10 ^ % { 4 } .
The contribution of starbursting objects is negligible .
In general QSO show r \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2.0 pt \hbox { $ > $ } } 10 ^ { 4 } and 0 \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2.0 pt \hbox { $ < $ } } b _ { J% } - R \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \sim$ } } \raise 2.0 pt \hbox { $ < $ } } 1 .
On the basis of the R magnitudes , we estimate the sample of radio galaxies to be complete up to z \simeq 0.3 .
We can therefore divide the whole sample of radio sources into a low-z and high-z population .
The low-z one includes the objects identified as galaxies in the APM survey , the high-z one includes sources either identified as QSO or with no optical counterpart for b _ { J } \leq 20.5 .

We find that radio galaxies are strongly clustered and highly biased tracers of the underlying mass distribution .
Models for the angular correlation function w ( \theta ) show good agreement with the observations if we assume a bias factor b \simeq 2 at z \simeq 0.3 .