Quote Originally Posted by jvarsoke
IANAL: I believe copyright on prints is dated to the moment of the print's creation, not the negative.
But, author rights for particular image are on image, that is on negative or in this case glass plates. If no other contract is made, author rights are valid up to 70 years after authors death. Authors right are transferred to its heirs (children, etc...) if author have them in moment of death. Of course different countries have different laws, but usually is like I described.

So, if glass plates are in your possesion, and you are owner of them, author and his/hers heirs still have their rights, oposite to owners rights you have Michael. But, as plates are made in 1890es, and if there is no other contracts or heirs of author, I think what jvarsoke said would be best way to go.

So, like museums title images: Image title, year of making, author unknown, printed by Michael Slade year of printing, copyright Michael slade (if authors rights are not on other person by contract or authors heirs)