Thank you Robert & Steven.
I've been too involved with academic hassle (as every first half of any week) to return to this issue.
It's very gratifying to read how, many scrap knowledge I had on the subject, is put together by your contribution.
I've done little progress meanwhile. My first setback was discovering that the Encyclopaedia is out of print
I was reluctant to resort to the Astrophysics Department in my University, but I'm reconsidering it. And, of course, I should raid the Library, just in case.
Anyways, please, keep contributing as you are doing.
PS: I'm starting to read Jacob's paper; it seems to answer half of my question: the distribution of the visible emission lines in PN. As I'm prone to jumping to conclusions (it's builtin, I can't avoid it, lol), please take it with as many grains of salt as convenient.