If you bought the NEGATIVES, I would guess you bought all the rights as well. Just don't call them your work.
Not really...
IF you bought negatives, you boghts ownership onto negatives, NOT author rights. Like in magazines for example. Magazine is owner of photograph/negative/silde, but actuall photographer (who works for magazine as employee or on contract) keeps author rights over photograph.
And how long author rights are valid depends of particular country. In some it is 70 years AFTER death of author (not after making particular work/photograph, but after death of author of that work, do not make mistake on this!), in others it is 90 years after death of author, etc... And then heirs (children or grandchildren or others) inherit belogings of thiere ancestor, in this case work of author. Then, maybe author transferred rights to someone and/or heirs of that third party, and maybe there is written trace of that. Many things can happen...
So, consulting attorney would be good thing to do...
For example in my country, law say next:
Author have rights over his/hers work by finishing that work, Author DO NOT HAVE TO REGISTER anywhere that work as its work. BY making particular work author automaticaly get rights over that work. Authors rights over work is valid 70 years after author death. Author CAN NOT GIVE UP of author rights. He or she can give some rights to other parties too, but can not give up of his/hers rights. Even for works author made for others, author automatically have rights over works, no mater if owner of works can be party other than author itself. No matter in what position author is regarding owner of that work. For example author is photographer in some magazine. In that case, magazine is owner of photographs, have rights to publish, maybe to sell to others photographs, but actuall photographer still have author rights over photographs, and author must allways be mentioned as such. Author can even stop magazine to use photographs, in some circumstances.
Again, as I am not expert in law, this is only what I read in officialy published document of my government regarding authors rights, I can not give you advice. I would consult attorney who is expert in those things...