Several surveys that make use of various detection methods have been carried out in the optical band. Some of these have been tuned to the search for BL Lacs. However, despite the large efforts only a handful of BL Lacs have so far been discovered at these frequencies.
2.2.1 The Palomar-Green sample
The Palomar-Green (PG) sample covers 10714 deg2 of sky down to an average limiting magnitude B = 16.1 for U - B < -0.46 (Green, Schmidt & Liebert 1986). Four BL Lacs were initially identified in the catalogue: OJ 287, OQ 530, PG 1553+113, 1H 1219+301 (2A 1219+305), the last object not belonging to the complete sample. Three more objects, originally misclassified as white dwarfs, were discovered by Fleming et al. (1993) by cross-correlating the PG white dwarf list with the RASS. To check if there were other known BL Lacs still lurking in the catalogue we cross-correlated the PG sample with the present BL Lac catalogue. Much to our surprise, we found that Mrk 421 was only about 51 arcsec away from PG 1101+385 and PKS 2254+074 was about 9 arcsec away from PG 2254+074, the former being classified as a ``composite spectrum object'' in Green et al. (1986), the latter, which does not belong to the complete sample, being unclassified. While an offset of 9 arcsec is consistent with the typical accuracy of the PG positions (about 8 arcsec in each coordinate), the larger deviation of 51 arcsec could be due to the fact that Mrk 421 is an extended object. Indeed, an examination of the charts in the PG catalogue shows that PG 1101+385 coincides with Mrk 421 (R. Green, private communication). The total number of BL Lacs in the PG sample is then 9 objects, of which 7 belong to the complete sample. We cannot exclude the possibility that other BL Lacs are present in the sample awaiting discovery. The PG sample gives only a lower limit to the number of optically selected BL Lacs, since many such objects have U - B colours above the selection limit: using the colours tabulated in Véron-Cetty & Véron (1993a) and Hewitt & Burbidge (1993), to derive the U - B distribution of known objects, we estimate that the PG sample misses about 40 per cent of BL Lacs.
2.2.2 The optical variability sample
A survey of optically variable quasars over 18 deg2 was carried out by Hawkins et al. (1991). This led to the discovery of two BL Lacs brighter than B ~ 19; some more could be present in the field. V magnitudes for the two objects have been derived from their mean B magnitudes and B - V colours.
2.2.3 The optical polarization survey
An optical polarization survey covering 560 deg2 of high galactic latitude sky was carried out by Jannuzi, Green & French (1993). Only one BL Lac candidate was found and the conclusion of this work was that most BL Lacs are not highly polarized (e.g. they do not spend much time at Pmax 30 per cent) and that much higher sensitivities in polarization levels are needed to detect a significant number of new BL Lacs.