Conclusions

We have shown that:

• Nuclear H II regions, Seyfert 2s and Liners lie in distinct, well separated regions in the log($\lambda$5007/H$\beta$) vs. log($\lambda$6583/ $\rm H\alpha$) and log($\lambda$5007/H$\beta$) vs. log($\lambda$6300/H$\alpha$) diagrams. There is no continuity between Liners and Seyfert 2s, with an apparent deficit of objects with $\lambda$6300$\lambda$5007 = 0.25.
• A number of objects have ``transition'' spectra, falling outside the regions assigned to the three types of emission nebulosities. They probably all have a ``composite'' spectrum.
• We have isolated a class of Seyfert 2 galaxies with weak [N II] lines. This weakness could be due to an under-abundance of nitrogen or to the presence of matter-bounded clouds in these objects.